I just did this last weekend, had about quarter of a quarter-keg of yuengling remaining from a lil crab feast.
I kept the beer iced for 3 nites. after first nite , about 10hrs after last pour/pump by hand, the beer was completely flat - no good. I then put it in a 1litre ss bottle and presto - tasted just as before - indistinguishable from the beer at first keg tap! Caution; did over carbonate one time though - was like extra dry champagne -that one - no good! But no need to throw away keg beer anymore, just keep it cold and reanimate w/ ss. warning can foam out some so expect that and let it settle and unscrew slowly.
someone - maybe me - needs to demo on youtube - a serious demo.