i don't remember the time, but i'd actually guess i was there for more than 15 min. i remember shaking and listening and thinking that i'd been there a really really long time and i could still hear gas going in. as i said, i did it until the shaking no longer made the gas hiss.
when you shake, do you have the keg upright and shake it like a british nanny, or in another position? i had mine on its side on the floor and i was rolling it back and forth pretty vi-gorously to make sure it sloshed a lot. [maybe a 1/4 roll in each direction in quick succession.] i was trying to keep the gas in post on top so it didn't dip below the level of the liquid too often. [that seemed like a good idea, for no reason.]