Maximum set of stairs users are willing to climb before needing an elevator

I know this question has many variables related to ones own ambition / physical condition, but I'm wondering if you were to have a multi story building, what would be the maximum set of stairs a user would be willing to climb before needing to install an elevator? My thought was most users would favor one set ie a two story building, but the limit would be two sets, or a 3 story building. Any thoughts?

Lets also assume our user group is young to middle age healthy individual without any disabilities within an apartment building. The thought behind this question is how many floors can you have in an apartment building without having high turn over due to a lack of an elevator.