That's odd, I haven't had to re-login in a long time. Do you know how to check your cookies in your browser? you can see which cookies are being used by TTOL and when they are set to expire. Perhaps they are getting deleted from your browser rather than timing-out. My "logged in hash" for "ubbt_hash " has a 1-year expiry; so I believe you can't blame the forum but need to look elsewhere for the cause.
I don't know if it's the browser (I use Chrome), but I have the same issue on all the forums I watch. Some of them are based on the same forum application, but only a couple. In all cases, they require me to sign in after not visiting for a while *on that computer/tablet*. Even if I was on this morning and went to a different device where I had not been on for a while, I will need to log in.
So, it seems to be the browser (not sure if all work this way). With Chrome there is no expiry date for the password cookies (as far as I can tell).