I have found Alclad to have a pretty good shelf life. The only bottle I've had go bad was about three years old and only an eighth of an inch left in bottom.
I have had some contamination problems getting paint thinner mixed into it. That sure ruins it. If I have been using enamels and clean a bottle with paint thinner I have to be sure it is dry before putting in the alclad. I also make sure the airbrush is thoroughly empty of paint thinner. I run lacquer thinner through the brush for a minute or so.
Because Alclad is so thin, I find I have the needle very close to closed, so it needs to be very clean. I find a slightly dirty brush that still works with enamels needs to be cleaned before using it with alclad. Also, if I am doing a long session with alclads I may have to stop and clean it a couple of times (on the other hand, I find alclads clean out easier than enamels).