There's never really a great time to leave a busy company, and the suck it creates is just the normal suck of being in business. As an employee, they stop paying you when you stop showing up - that's the deal they signed on for.
But I think what makes for a good decision depends on a couple of things:
a) If you are completely miserable, go.
b) If you meaningful experience bringing similar projects, to market, go.
c) If you don't have significant experience bringing projects like the one you are working on to completion/market/whatever the deadline is, consider staying because the experience you gain may be worth it. So might the reputation. It's just looking like a year, after all and you might get better at implementing other people's ideas in addition to your own.
If you leave, and you are the only developer, and there's real money on the line, hard feelings are pretty likely. Whether that's going to burn important bridges or not, depends on the players and what you wind up doing over the long run.
Good luck.