I have polished the internals, sweetspotted the reg, and use VERY good paint to barrel match (Freak Stainless and a Deadly wind barrel).
I get about a hopper (120 rounds) per 500 psi of tank pressure. The marker runs on 300 PSI nearly exactly.
So - 4200 PSI divided by 500 psi equals about 8.5 pods. 8.5 x 120 = 1020 rounds.
A little over half a case.
This means on a 68/4500 gets about 1500 rounds or so.