Plug and sawdust spawn will store for six months to a year in a refrigerator. The fresher the better, but mycelium is pretty resilient. If you don’t get around to inoculating right away, tuck it away in your refrigerator until you’re ready, with the exception of Pink Oyster Spawn which can't tolerate cold temperatures.
Mushroom spawn will naturally develop a layer of white mycelium (or yellow for Chicken of the Woods). This is totally normal and indicates healthy spawn. Break up your spawn bags thoroughly before use. And be sure to use logs cut within 4 weeks of inoculation.
If you're inoculating logs, be sure to inoculate your logs within 4 weeks of cutting them. We recommend avoiding log harvest in the spring between bud well and full-leaf out, since nutrients are directed toward flower and foliage growth and bark is loose. See our section about best time to harvest logs for more info: When to cut wood for inoculating logs