How is the duration of stay in a location determined regarding the ruling of travel or residence?

Primary text

If the husband remains in a town for a period of twenty-one days of prayer or less, this duration is not counted against the initial travel arrangement, as it falls under the ruling of travel, and travel regulations apply. If the stay exceeds this duration, he must fulfill the obligations to all wives for the time spent there, as he has exited the ruling of travel.