As Ursula says, they vary a lot. I think most of the Phals you see in the shops here should last 6-8 weeks, though often will last up to 3 months. Cymbidiums (the other common genus here in Melbourne) will typically last around 2 months, depending on the hybrid makeup (the shortest-lived flowers on a Cymbidium species that I know of only last 10 days).
