If you don't hit the "Remember Me" button when you are signing in, the site will sign you out after 30 mins of zero activity. Activity means clicks, not typing. If it's kicking you out after 15 min, then my guess is either A)the cookie file that keeps you logged in is being rejected or B)the cookie file is corrupted and needs to be reloaded.
A) would require you to have edited your settings. If you haven't, then it's probably B, and in that case best approach would be to clear your cookies and cache from your browser for the site and see if it keeps happening. If it does, come back to me and let me know what browser you are using (testing with Chrome over here)
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