The forum system will keep you logged in for one hour after your last activity (clicking a link or button). If you are composing a message that is taking you a while to complete, it is a good idea to click on the 'Preview Post' button that is next to the 'Submit New Post' button. Doing so will update your activity timer and keep you logged in while you complete your post. It will also give you a chance to see how your post is shaping up, and correct any problems with tags.