For beach vacations, seven to eight nights is perfect and enough time for me to relax into a trip. I will also do a five night long weekend if a getaway is urgently needed, or if my budget truly can't stretch to get the room category I want for seven nights.
For extended sightseeing trips, like Peru, a two night minimum per location is a must and I like to include a couple of three night stays to relax the pace a bit and offer some downtime.
My husband hates switching hotels, in part because he loves to fully unpack immediately upon arrival. I am generally fine with living out of my suitcase, and switching hotels is par for the course with my career. However, I do my best when planning our trips to ensure that we stay put in one location as long as possible even if it means less efficiency for sightseeing.
Flights can also dictate my length of stay, especially for beach vacations. For example, we have been wanting to get to Zihuatanejo, but when I look at the flight options from BOS if we don't go Sat-Sat then it requires two stop flights. So if we have the time to go for seven nights that's fine, but we wouldn't do eight nights, and less than seven is not appealing given the travel hassle. Similarly, if we are going for a long weekend, a nonstop flight and the schedule itself can drive the final decision on destination and duration.