I would not wait more than a week, that way I could back date it to when I found it.
My preference is to know excactly when & where someone found my bill. If I stamped a bill on Monday, & you found it on Tuesday, it would be my fastest hit, but if you waited 5 or 6 days to enter it, it would be just a hit.
So if you want to save entering a hit for a few days, just back date it to when you got it.
I think this is the long answer!
My problem is I have never come across another marked bill, since I started here, I gave one hit but it was a Stealth.

Mike Cityscape