Something I always did when I first got into polaroid photography – more precisely, peel apart film – was time development exactly. Most colour peel-apart emulsions are designed to develop after 2 minutes in 20 degree ambiant temperature. A bit longer if it’s a bit colder, a bit quicker if it’s a bit warmer. But I always looked at my watch and timed it exactly, and when the time was up, I peeled the film to stop development.
Well, that was all unnecissary. Try it yourself, but it really makes no difference when you peel the film. The development process used for polaroid film is what’s called a monobath. A very very complex and well designed monobath, but a monobath. It means that it contains a very quick acting developer coupled with a quick acting fixer, mixed into one. The image is developed by the developer components, and the fixer takes over not long after, finishing the process. So development is stopped by the chemistry itself. Which means it doesnt matter when you peel the film at all, the times listed are just the minimum development times.
So don’t stress about it.
EDIT: Looking back this was a bit of a blanket statement to make. I believe some of the earlier polaroid peel apart emulsions could be damanged by developing for too long, but with Fuji FP100B/100C and FP-3000B, the only peel apart film currently available new, I’ve noticed no ill effects by extending development by up to an hour. After 8 hours, FP-100C has shown the emulsion to soften though. So, dont stress about it, but exercise some cuation.