That does seem to be the nature of the beast of PVC glue. George's idea may be the best. FWIW, I too like Lew, put the cap on real tight. Actually helps if you use the glue to "glue" the cap on. You have to use pliers to open it back up, but seems to increase the shelf life. I usually buy the smallest can possible unless I'm doing a big job. Usually my can is still good for a few months. Seems that extreme temps make it worse. If kept in a conditioned space seems to help also.
Or, you could just buy a new can for every job and bill them for it.
