I would say between 50-60k....it was initially down as 80k, but there was alot of feedback on belts snapping gone 60k, if you are not sure if the car has had one done and you are over that mileage, get it done asap.
I had mine done at 55k and then another one at 110k....also if its the 2nd one or gone over 80k, it might be worth getting the dampener/tensioner and waterpump all done at the same time as well.