There seems to be some confusion or imprecision in terminology. In conventional usage, "cask" and "keg" are two quite different things. Both can be SS. A cask is a vessel in which a porous spile driven into the shive allows CO2 to exit and air to enter the cask. It is used to condition and serve ale at a minimal level of carbonation, without dispensing under pressure; because it is exposed to air, the beer must be consumed within a few days. A keg is a sealed pressure vessel which can be used to condition beer by refermentation (just as in cask or bottle conditioning,) but more usually is used to hold and serve racked and force carbonated beer. I wonder which the OP really intends. If he means he will be using casks, then the beer will in fact not keep at all after it is tapped and the shive pierced. If kegs, then as long as it is kept at a cool temperature it will store well for months, but maybe not as long as in bottles because there is more opportunity for oxygen ingress at the keg seals than at the bottle cap. As always, how susceptible a beer is to aging is dependent on the style. If kegged beer will be stored for any extended period of time between serving sessions, disconnecting the gas and beer lines will reduce the opportunity for oxygen ingress through the permeable tubing.