Do you mean there is a one year commitment if you win? Or do you have to stay in Boston for the year?
It runs from June 27 through Oct 1. Final judging takes place in the first two weeks of October, and the final ceremony is on Oct 20.
Founders do not need to be present for that whole period, but we do strongly recommend active participation -- surveys and data demonstrate pretty clearly that startups perform much more effectively if they engage in the accelerator rather than "dialing in".
There is zero obligation to stay in Boston following the accelerator.