It looks like for an IMO compliant display, the lost
target icon displays when there has been no report for the shorter of 6 minutes or 5 intervals. Both Class A and B devices report position every 3 minutes when stationary which is the longest interval. The 6 minute report interval, BTW, is for static data like Ship's name, and is the same for Class A and Class B.
I don't know whether consumer devices follow the same rules, but I can't see why they wouldn't.
Again, on an IMO compliant device, lost
targets only go away when explicitly dismissed. I find in practice that they just disappear from the screen and are out of sight, out of mind.