I selected this particular question this week because it speaks directly to a common misconception that a number of junior associates have regarding practice-area switches.
One particular comment in your question caught my attention: "...how long should I wait before I disclose to the litigation partners that I plan to switch to the firm's corporate group." This comment stuck out because your use of the words "disclose" and "plan to switch" implies that you are expecting that your request will be automatically granted.
Practice-group switches certainly do occur, but junior associates in firms (typically larger firms) too often believe that a practice-group switch is simply a matter of making a wish and having it granted. Why? Because in many firms with summer associate programs, summer associates are allowed-and encouraged-to chose their specific practice areas. During law school, students are encouraged to explore different fields of law and sample different areas. A mindset that embraces exploring and changing is encouraged in law school.
When you graduate and join a large firm, it's different. With the exception of firms that rotate associates during their first year or two, firms will generally hire you for a specific practice area (e.g., litigation or corporate). There is a business need that justifies placing you in a particular group, and the firm is expecting that you will be able to help service specific clients in that particular group. Placing you in a particular practice group is usually a well-thought-out investment decision that has been justified on a number of levels.
It's also important to realize that practice groups within firms often operate like separate subset firms, and many have little or no cross-pollination. Many junior associates have the misconception that just because a firm has, for example, litigation and corporate practices, they will have the luxury of freely switching practice groups since they all work for the same firm. This is not true. It's important to keep in mind that firms view associates as investments. For each junior associate, thousands and thousands of dollars are invested by the practice group-through recruiting and training-and the firm does not reap financial gains immediately.
With this background in mind, let's answer your question. As you will likely guess based on my comments above, I think it's a very bad idea to lateral into the litigation practice of another firm with the expectation that you can automatically switch to a corporate group. Is this a possibility? Of course. But don't expect it.
If the firm has hired you for its litigation practice, it is expecting that your background and experience will make you a good fit for its current needs. Based on the fact that you did not disclose your interest in transitioning into corporate law, the attorneys there are very much expecting-or at least very much hoping-that you will fit nicely within their litigation group for a long time. They will be excited to have you join their team. If you unexpectedly spring your interest in transitioning to their corporate practice on them, they will likely feel deceived. They will wonder, "Why did we invest the time and effort into hiring somebody for our litigation group who had no intention of staying with us?" Aside from their disappointment in learning that you are not who they thought you were during interviews, you will lose a lot of credibility in their eyes.
Second, let's assume that the attorneys in the litigation group have the personalities of robots and don't mind your request to transition into the corporate group. How do you know that the corporate group will have a need for somebody with your background? Unless the corporate group specifically has a need for a lateral associate with no relevant experience (which is very unlikely), this door will likely remain closed, even though you are working for the same firm.
Now-for the sake of argument-if you were coming from a practice area that provided you with more transferable skills (let's say finance, for example), you might have an easier time making the switch. Remember, when firms hire laterals, they expect to hire people who can hit the ground running (apologies for the cliché phrase) or people who can get up to speed quickly with minimal training. But as a litigator, you are essentially asking a firm to bring you on as a lateral and train you from the ground floor on corporate/transactional matters, which differ considerably from litigation. Assuming the firm has a lateral need, when given the choice between a litigator who joined the firm a few months ago or an experienced corporate attorney from another firm, wouldn't it make more business sense to hire somebody who already has experience in corporate matters? (I should point out that if you have been with the new firm for a while and have established a very good reputation, the goodwill you have developed could make it easier for you to make an internal switch, assuming the firm's need is for somebody with less experience.)
So where does this leave you? I'd recommend stepping back and seriously reconsidering your strategy. My hunch is that somebody along the way gave you the false impression that it's very easy to switch practice areas within a firm, which is simply not true. Again, it can and does happen (I've successfully placed a number of people into new practice areas), but it requires a detailed analysis and a very specific approach.
If you move forward with the strategy you proposed, I'm sure you'll be anxious to try to initiate the practice-group switch sooner rather than later. I'm concerned that you could risk hurting your reputation with the litigation group at your new firm, and your loss of credibility could undermine your entire experience at your firm. And if your desire to switch practice areas results in the firm asking you to leave (which I've seen happen before), you'll have a much harder time explaining this to your next firm. The people there will be thinking, "If you had decided that you wanted to practice corporate law, why did you join the litigation group at your second firm? That makes no sense." And if your new practice area does not work out, you'll have a tough time successfully explaining to a third employer why you are looking to make yet another switch.
Making a practice-area switch can be exciting and very rewarding, but it's not something to be taken lightly and definitely not something to be entered into carelessly. As mentioned above, there are numerous complex strategies involved in making a successful practice-area switch (which would be a good topic for another Q&A...hint, hint). Spend time now doing due diligence and speak to people who have successfully made practice-group switches. You may also want to consider the advice in the following article, which was written by a BCG Attorney Search recruiter who has helped a number of attorneys with practice-area switches: www.bcgsearch.com/crc/practice_area.html.