Average Length of Stay is the average number of nights a guest books per reservation. It's an operations metric as much as a revenue one, because short stays cost more to service than long ones.
ALOS = Total room-nights sold / Number of bookings
A property that sold 2,880 room-nights across 960 reservations has an ALOS of 3.0 nights.
Longer stays are cheaper per night to run. One arrival, one departure, and one deep clean spread across more nights means less front-desk time and less housekeeping turnover, which pulls down cost per occupied room. That's why a property will accept a lower nightly rate for a longer booking and still come out ahead.
ALOS is also the lever behind stay restrictions. A minimum length of stay on a peak weekend protects a property from a one-night booking that blocks a more valuable pair of nights, which is yield management applied to length instead of price.
Watch it by segment. A blended ALOS that holds steady can hide a collapse in longer leisure stays offset by a rise in one-night business travel, and those two want very different things from your pricing.