Good question. Without a gauge it would be hard to guess -- need some trial and error until you get it right.
When I've kegged in the past in the small 5-liter kegs, what I did was prime with like 1 tablespoon cane sugar, then let that sit for a couple of weeks. This would usually be enough pressure to push out 2 or 3 pints. Then after that, I would add "a squirt" or two of CO2 to maintain the carbonation until next pour. This worked well, but again, required some trial and error. If you tried this with 2.5 gallons, I would guess you'll need twice as much, maybe 3 "squirts" of CO2 to maintain. By "a squirt", I mean you're pushing the CO2 trigger for perhaps 1/3 to 1/2 second per squirt. Play around and see what works.
Somebody out there must have way more experience and can give you a better idea to start out.