Retarder comes in gallons most of the time, and a gallon goes a long ways. Chances are you'll through out the remainder at the end of the year. Shelf life depends. The thing to bear in mind on retarder is that it slows the drying process down, so naturally it will take longer to finish a project. It also thins somewhat, so you have to test a piece for running Another trouble maker are runs.
Runs happen for a number of reasons.
Spraying to close to the object. Staying in one spot to long, material to thin, humidity causing sag, or someone upstairs is paying you back for something you did, and so on. Have some masking tape handy, and while the finish is wet, take a strip and gently glide it onto the run and let the tape rest on it, then pull it off. This will take the run out, but it still has to be sanded and shot again. If a run dries, sanding it will cause material removal in all areas, an chances of bare spots or lighter areas surrounding the run are pretty good.