(1) If you can still smell the chemicals coming from the finish, it's not done yet.
(2) If you can dent the finish with your fingernail, its not ready to buff yet (select an out-of-the-way place like the back of the headstock to test this).
Most people think when they're done with the woodwork and assembly, they're almost done with the guitar. If you're planning on an oil finish, that's mostly true. But in reality, if you're doing your own painting and/or clear coat finishing, you're about 50% done - in other words, for a good spray finish you can end up spending almost as much time on the finish as the whole rest of the instrument.
Even after doing your homework, so much of finishing is trial and error, everyone who has stuck with it and strived to learn good finishing technique has probably stripped 2-3 times as many finishes as they've buffed out.