When you get a gift via email from MR, you must use it within 30 days or else it expires. Most of this is straight forward, you simply activate it within 30 days to use it. With Durden Mapping, you need to claim it within 30 days (Once claimed its now a durden credit that lasts forever)

If you do not claim it within 30 days, it will expire.

Regardless of the source, if you open the gift of Mapping, it becomes a credit waiting to be used during a spawn. Once a credit, it doesnt expire.