You will be logged out after a certain amount of inactivity (30 minutes on this forum, I think) or earlier if there is a forum glitch which seems to happen rather often.
As
explained in the FAQ, you can check the 'Remember Me?' box when you log in, so that you will automatically re-login after such automatic logouts. Only do this if you have full control over your computer so that nobody can steal your cookies, otherwise they obtain your VEF userid and a hash of your password.