Corsair offer a 5yr warranty on their coolers, so it would make sense they would last that long. Though it is always a possibility you receive a unit with a defective pump that breaks after 5 minutes or a whole year. You can never really say for certain if it will last.
But at the very least I would expect it to last 5yrs before it dies of "old age". If it does, then you get a brand new unit and warranty.
A custom loop is much harder to say because you have so many different component makers, there's the greater possibility of user error and for something to be used in a way not-intended. So it would come down to the quality of the parts used and whether the person building it knows what their doing.