Sessions Timing Out Too Often

I'm finding that my sessions seem to be timing out fairly quickly. For example, in a little forum I wrote with PHP, people are telling me that if they type a long message and click Submit, they are taken to the Login page because their session obviously timed out, and they loose their posts in the forum. I do have a "remember me" feature that uses a cookie, but, not all visitors are using it, and they are the ones experiencing this problem. Is there a way to extend session time, and is that the best way to reduce this problem from happening to most members? Thanks! Monty