This is something I inquired about before, and Mike responded with that anytime after an hour of inactivity, your last location will disappear, but that it wasn't exact and could take up to two hours. However, two hours was the maximum time it would take to disappear.
So, I checked my profile a few minutes ago on KH-Flare, and I saw this:
It's not really a particular "should". If the task runs once and hour and the clean up only happens when the record hasn't been touched for an hour, then it'll be removed between 1 and 2 hours (approximately) after the last page load. So basically, after 60 minutes it can disappear at any point but for the general case, it's not deterministic.