There is no time limit. If you maintain legal residency in the state that issued your multistate license and you remain in good standing, you may practice in other compact states/territories.
If you were to take an action (while practicing in another NLC state or otherwise) which would change your legal residency status (see example below), then you have given up legal residency in that home state and you must now apply for license by endorsement in the new state of residence. The new license issued will replace the former license.
For example, a nurse has legal residency in Arizona and practices temporarily in Colorado for six months on the Arizona multistate license. While the nurse is practicing in Colorado, her Arizona driver’s license expires. Rather than renewing the Arizona driver’s license, the nurse obtains a Colorado driver’s license. Because a Colorado driver’s license is only issued to a Colorado resident, the nurse has now become a Colorado resident unintentionally. Nurses must be careful not to take actions that would change their state of legal residency, when practicing in another state where they temporarily reside.