Some last longer, some die quicker, it depends on the conditions and upkeep for a particular installation. I threw out 70 years off the top of my head to demonstrate that they tend to last a relatively long time. It also depends on what you consider to be "replacement" vs. "repair". Sometimes all you have left is a concrete shell with rusted out turbine husks, in other cases you just need to replace a few parts. Hydroelectric generation (and water-power in general) has been around for a pretty long time, and there are a lot of different local implementations out there when you get into the details.

The badger hydroelectric plant in Wisconsin is 86 years old, and that's just one I happened to know because I collected data on it recently. If you go further east in the states and then across the Atlantic, the plants can get older than that as well. http://www.kaukaunautilities.com/about_us/default.asp?Catego...

EDIT: Oh, and in terms of cost, when planning out capital expenses most hydro projects are assuming a shorter lifespan than actual. I don't have numbers in front of me, but I feel like costs are generally accounted for a 20-40 year lifespan (weighted towards 20), although they do expect the plants to operate for 2-3x as long.