Not sure exactly how many old sites I've hunted that I have found an old coin or two, but it's far more than 100. Here is sort of how it plays out here.
I'll put my sites in groups for timeline because they do tend to be quite different.
Sites dating about 1820 up we still find the odd 1700s George copper. Early 1800s would be common, 1770s George III the odd time, but probably wouldn't see too many George II. I wouldn't call it rare to see a II, just wouldn't expect many.
Sites dating 1780-1820 George II and George III coppers are normal, the odd King George I does turn up, very rarely a William III (1695-1701) of which I can only confirm 2, and I have dug a lot of coppers. I have never found or witnessed a William & Mary (1694) and all Spanish are milled, never a cob from these sites.
My earliest date from 1720-1755. At these places Spanish silver can be both milled or cobs, but I have found more cobs. The majority of coins are 1700s but 1600s do turn up quite a bit. My oldest is a 1500s coin that was restruck in 1692, but I have witnessed a 1300s English silver.
So here it is possible to find coins quite a bit older than the site, and it does happen quite a bit, but to find ones that are much MUCH older you still have to be a bit lucky. When my friend dug the 1300s silver penny I was pretty surprised and that was thinking it was probably mid 1500s.