The site does time out !
Once upon a time, it was 5 minutes, then it was much longer, and now it's shorter again.
Assume that it will time out.
I keep a second window open to my acount, just to refresh and log back in before posting or sending anything, else the site will probably do exactly what you are seeing.
The other thing you can do, especially if what you're writing takes time, is to compose it off-line, sorta, in notepad ( do NOT use word !! ) and when you're ready, log back in, then copy/past your composition.
how long it takes to time out just depends on the day of the week. there are times i get logged out if i spend 30 seconds reading a post other times i can take a vast amount of times to post pictures in a post without getting logged out.
As well, I have just recently discovered that a whole bunch of issues for me are solved by using Firefox as opposed to Explorer. Maybe it's my settings but at least Firefox properly uses quotes, doesn't take two tries to post and uses emoticons properly.
Try that if you're having troubles like I was.