Since the move to https, I have to either sign in if I have been "inactive" for more than a certain period*, or refresh (f5) frequently to keep me logged in.
*The period seems to vary depending on the time of day. In the mornings I have to refresh more often if I am not seen by the site to be active. This includes answering posts. It's frustrating to write a detailed reply to a post and find that I am logged out by the time I hit the Submit Reply button. I have learned to copy my reply to the clipboard in case my work disappears.
it's okay most times because the site remembers my draft when logged back in (as long as I haven't left the thread to which I am replying). On the odd occasion that it doesn't I have to start again from scratch unless I have my work copied to the clipboard.