If we were to make an offer, when would you be able to start?

Be honest and professional in your answer. If you have an upcoming bonus and have to be at your current company at that time to get it, let your interviewer know. Otherwise, if you have no other commitments, then providing a standard two-week notice is appropriate.

A side note on this question is to be sure to present the idea that if you accept an offer and your current organization immediately terminates you upon your two-week notice, you may be able to start sooner. This is a way to decrease the possibility of your being unemployed betwee the two jobs.