For me being logged in has never lasted even a full day. I used to complain about it in 2007 but I've learned to live with it (it's never been modified in the last three years, to my knowledge).Now my login lasts exactly one page. It was holding for a bit a while back. now simply till the next page visit. eh well...
Solved by deleting cookies again. Now it looks like login lasts a little. Still, seriously, no need to put a timing on a login, imho.
I am logged into IMDB for a year, for example.
The problem is that it seems the access is reset every time something changes. Not just IPs (which doesn't make sense) but also web browsers. Coupled with the innecessary ajax-y username box that requires an extra click and doesn't allow a lot of browers to memorize the username and pass it's always a PITA.