I leave it until it looks like there's nothing good left in the pulp. I use strainer bags, so it's easy to lift it, let it drain, then discard.
This varies according to the fruit used and how small the chunks are. Mashed berries don't need nearly as long as chunks of peaches or apples. The timing also depends on how much skin extraction you want for color and tannins and the likelihood that seeds, if any, will add bitterness.
This is something you'll need to develop you're own preference for over time.
"To squeeze or not to squeeze" is another question. Some people really wring out the pulp bag, others gently squeeze and others don't squeeze at all, letting the bag drain on it's own. IMO, this all depends on the condition of the pulp and whether or not you feel like there is something left to squeeze out of it. Most times I just let it drain, but occasionally I can't help myself and give it a gentle squeeze to speed up the process.