Just to clarify, the 24hours indicated above is dependent upon the SEP Client on the machines no longer communciating with the SEPM (i.e SEP uninstalled or machine off), and the computer account having been removed from AD and the synced OU (which will eventually result in the client record being removed from the SEPM's DB after the next AD sync).
The client record being removed from the SEPM's DB is the end aim, as only then will it release the license.