Thanks for the question, I'm typically using Chrome. I might go 2 or 3 weeks between visits to the bitbucket web page, and I've noticed that when that happens I have to sign in again. I also go a couple of weeks between visits to github and other sites that require a login, so I'm assuming its an issue on the Bitbucket side (probably the lifetime of the cookie).
It could be that every visit to bitbucket "refreshes" the cookie for another 2 weeks. I haven't look real closely at the issue.
Ok, this is funny... I clicked on the link in my email to get back to this notification... and I was logged out of Bitbucket! The only thing that had happened in the last 8 days was I did a reboot of my machine. I'm still "logged in" on other sites like github, twitter, etc. But with Bitbucket... I have to sign back in to leave this comment.