How long is the fuel cell used in cars (like the Hyundai Tucson Fuel Cell or the Toyota Mirai) expected to last in terms of mileage or time? What kind of loss of performance is expected, like lower power or lower efficiency or total failure?
There are two things you need to be interested in: calendar life (which should ideally be 20 years) and usage life (which should ideally be 1 year). A car has about 5% usage time, the rest of the time it spends idle.
The calendar life can't be tested because automotive fuel cells are a relatively new kind of technology. However, I would trust more on the calendar life of fuel cells than on the calendar life of batteries of electric vehicles. There should be no continuously occurring chemical reactions in an automotive fuel cell.
The usage life is where I think the real limitations might be. Manufacturers certainly have the information about the usage life, as it takes only 1 year to test whether the usage life is acceptable. The big challenge is obtaining that information. I'm fairly certain the manufacturers don't want to make exact details public.
However, consider this: Toyota has for a long amount of time been considered to make the most durable cars available. Would Toyota be so focused on fuel cell vehicles (even to the point of overlooking battery electric vehicles) if the technology wasn't durable enough or if there isn't a fair chance it could be made durable enough in the future?
The fuel cell in Mirai is Proton-exchange membrane fuel cell, and according to this Wikipedia article and the Fuel cell article, there may be some durability problems, e.g. automotive fuel cells are rated only for 5000 hours of use, about half of what I consider acceptable.