I "bung" it imidiately after filling with the Blichman Beergun.
Then back to the fridge at 3 C.
I have recently 2 5L kegs in the fridge unopened i 3 month, while collecting different beer to a party.
I once had one open, fitted with the delux tap, in my fridge for 3 weeks with no problem, and i could probably have stayed there several more weeks.
If the keg is in ok, no rust, no scratches etc, then the "shelf life" of the beer in the 5L keg, is more dependend on the cleaning and sanitation of the keg, pipes, hoses, etc. than the keg it self.
I rince the kegs with warm water instantly after it's emptied.
Before use, I clean them with a mild caustic solution, rinse, and sanitize them with an iodine solution, and rinse.
Al equipment is either boiled or sanitized before use.