I'd imagine that pressuring you to keep any really depends on the shelter. For the most part, though, we foster semi-feral kittens with the mutual intent of getting them tame enough (while they're still little/cute enough to be instantly adopted) to be returned to the shelter and adopted. We've never been pressured by any shelter to adopt them, but we've also fostered most through a no-kill shelter.
We usually keep them two to four weeks. Generally, the older they are the longer it takes them to tame up. Our first foster was a full-grown cat and it took him about six months to really come out of his shell and be friendly, but kittens normally tame a lot quicker.
It's usually not hard to let them go. They're little and cute, but after four weeks of having a ton of little furballs around it's nice to go back to a relatively quiet household. Letting the foster cat go, though, was really hard; it's hard to form an attachment after a month (especially knowing they'll be snapped up quickly) but knowing you're responsible for an adult cat going from "hiding under the bed and being terrified" to "waking you up in the morning with purrs and head bumps" is a bit of a wrench.
We normally get foster animals every two or three months. Generally we just drop by the shelter to say "hi" to friends, and there'll be a group that needs to be fostered. If you're interested, I'd just contact a couple shelters in the area and see if they need anybody. Depending on the shelter, it'll vary from "Sign these legal disclosure forms; we'll get back to you in a couple weeks" to "TAKE THIS KIBBLE AND BLANKET AND GET THEM TAME".