The natural time for mason bee adults to emerge from the cocoons is normally 
quite early, in February or March in the lower part of Vancouver Island. This year I decided to hold some dormant in the refrigerator for a longer time to see if they would still hatch out successfully. By early June some of the bees were actually emerging and remaining very still in their jars in the refrigerator.
As soon as jar was removed from the fridge, once they heated up they would fly off. (Note: as of June 25 they are still very active around the nest boxes.)
After a week of leaving the cocoons, I isolated the unhatched ones in a sealed jar, and sure enough in a few days the parasitic wasps were on the inside of the container.
Each time i had made a release of cocoons throughout the spring, I waited for at least a week and if they didn’t hatch, I opened them with a razor blaid and released any live bees. about one in 30 unhatched cocoons would have parasitic larvae, so maybe they are programmed to emerge more than a week after the regular bees emerge, in that way, they would have active cocoon building so they could immediately begin laying eggs in the cocoons. ??
NOTE: I have updated this 2015 post in order to make it appear closer to the other mason bee information