As we all probably have heard, two weeks is the industry standard for giving notice. Any new employer will absolutely expect and want you to give your current employer two weeks’ notice because they would expect the same professional courtesy out of you. However, in your situation it sounds like you are concerned that two weeks wouldn’t be enough. I would suggest you sit down with your managing partner and let him know the situation. Then ask him, in his ideal situation, how long he would like you to stay for the transition. Obviously, he will likely want you to stay indefinitely, but let him know that you are trying your best to work with him and make it a smooth transition, but also that you are eager to start the next chapter in your career.
I would say that 99% of the time, my candidates give two weeks’ notice. However, I’ve seen a few candidates give three weeks’ notice and recently had a candidate give five weeks’ notice to wrap up a particularly large deal that was closing. In order to keep your new employer happy, make sure this plan wouldn’t cause undue hardship on their end and explain your circumstances. It’s very important to keep your references happy and appease the partner you’re leaving if possible, but not at the detriment of your new position.
So, it will depend somewhat on your particular situation. In almost all cases, two weeks is appropriate and expected. However, there is some wiggle room when needed!