I've heard that, on the beach, 1 in 100,000 people drop gold...you could use this as a ballpark figure that, along with park attendance records/analysis could give you an idea how long it might take.
Coins replenish quickly, but the goodies can take some time. Plus, sometimes others may be hunting the site unbeknownst to you which would make any calculations meaningless. I've hit some high traffic areas dozens and dozens of times (like volleyball courts)...99% of the time there's nothing, but every once in a while something decent pops up.
Once you've hit an area like a tot-lot over the span of a year or two you will get a feel for what to expect.