Mine used to last 1 week. Yes, setting lotion or a leave-in increases the duration. What I would find was that the set would loosen a bit each day, but my hair was always "set" perfectly.
I just experimented this past weekend with the setting lotion. Yes it does increase the duration of your set. With setting lotion 4 days without not even two - my hair was looking sad this morning had to pin up in the back.
I usually wash after 1-2 days bc of excercising.
When I'm in the mood to wear my hair out, which is not often, the set could easily last 3-5 days. Anything past that, my hair will start to frizz.
A week..but it really depends on the weather. In the summer I may only get one good day before the humidity ruins it. That's why I mainly do them in the winter.