serving pressure seems to run around 5-7 psi. usually.
You said it's chilled, but get it really cold. I've been taking mine
down to 38f. Takes about 2 days in my freezer, all the while
I hold 30 lbs on the keg.
Anyhow, I'd say hold the 18lbs on the keg from the Co tank for
up to a week. release the air from the keg, and reset the gas to
serving pressure....see if that's what you'relooking for.