R.J.
My son's first year we were in 3 apts, countless hotels, and quite a few car trips -aka sleeping in hotel rooms at night, car in the daytime. The monitor was one of the first things to go, because it was useless. He was always at arms distance. The only time I broke it out again was at age 3 when we moved into a townhome and I wanted to workout outside while he slept.
So I would say; Whenever it stops being useful. :) No matter when others stop or start, usefulness varies for each family and each situation. A 200' hotel room, v 800' apt, v 2500' house v, etc., etc.,