Some forums have options in the backend where you can basically have a "never time out"
This is usually implemented on the client side as a cookie. The length of the timeout may be configurable by the software running on the server. All cookies have an expiration whether it be a specific time or when you close your browser. One cookie this site set on my computer expires in 2038 which is equivalent to never expiring.
I'm trying not to get too technical in my explanation because this knowledge is part of my day job as developer/system administrator.