Is there a way to edit the amount of time you stay logged in?

I'll be on the forum, and if I decide to make a new post I can't take my time in doing so because at some point I'll no longer be logged in. So, I'll have type up a long detailed post, hit submit, and then get taken to the forum index because I was no longer logged in. It would be nice if it would prompt me to log in again, and then post the message, but it doesn't. So I basically lose everything I typed up.