There are different tiers of expertise in archaeology. The tier depends on the discoveries you have made. You get 1 "Discovery Point" by smashing the rocks inside each site. With every tier you get access to more Pokemon and items. You can verify your tier by talking to Gingery Jones in Glacial Site.
If you encounter a spawn you're not "expert" enough to get, the Pokemon will flee or the item gets destroyed. You'll however gain a "Bonus Discovery Point" each time you'll encounter a Pokemon who fled or if the item gets destroyed. As the range of the bonus may differ according to the tier of the encountered Pokemon, I won't specify the said bonus.
Every sites of excavations have 25 or 26 rocks, except the new one with 18 rocks. It seems there is a bonus discovery point when you are not an "expert". But i will go for a total of 170 rocks because i don't know exactly the probability for flee or destroyed and the bonus discovery point.
I think 6000 discoveries is the max useful discoveries to have. 6000/170 = 35,29. 35-36 days to unlock everything.After 6000 Discoveries you can revive Kanto, Hoenn and Kalos fossils by talking to Gingery Jones for 10k per revive..