One of my other "hobbies" is making soda. I use the tire valve stem into a 2l bottle cap method for carbonation.
I usually set my regulator around 80 PSI to carbonate the soda. My regulator is a cheap tool regulator, so I really don't know how accurate it is.
As for of filling a 2l with pressurized CO2, I doubt it would last more than a day. You probably would get the same effect as tapping a bottle of seltzer water.