It varies by school and chapter. Personally, I lived in 3.5 years (second semester freshman year until I graduated). I would definitely do it again.

The most important thing, budget-wise, is that the house be full or very nearly full. At schools where the chapter house is smaller, it might not be an issue to keep a house at capacity every year. At other schools where the chapter facility is larger, members may have to live in longer than 1-2 years to fulfill their live-in requirement.

ETA: my chapter's house was (is) a "traditional" style home. Members could choose to live in 2-, 3- or 4-person rooms and every member slept in their room. There is a large kitchen and dining room area, two different areas to lounge/watch tv, a formal living room area, quiet study area and a computer room. It was like living in a regular house, only you had 60+ roommates.