It depends much on person, browser and settings.
Browsers usually reserve a certain amount of disk space, like 12 MiB, to this task. If a user stops using the browser it is indefinitely. If they uses the browser rarely, it will be until the expiration – either by internal policy or by HTTP headers. If they uses the browser heavily, it can be 12 minutes or even less.
I believe that it is hard to say ‘in general’ as I had a website on which 50% entries were from Firefox and 49% from Opera when IE at that point had over 75% market share. If the target audience of your site are users who use their browsers heavily, it can be very short. On the other hand if your site is only website visited it can be nearly never.