I beleive the volvo recommendation is six years or so many miles. The timing belt change interval on these cars are all different so without the manual it's hard to say. All the old red block motors were 50 000 miles. These white block ones are all over the place, the shortest was the 960, then the 850, next the s90 and longest the s70. I think the s60 are the same as the s70 but I don't own one and they may not be old enough to know for sure. The white block angines are interferance (the pistons will hit the valves and make a big mess) so it is better to change it early than late! a couple of hundred dollars for the belt is far cheaper than a few thousand to fix the motor! I have to say I agree with your mechanic on this one. He's tellling you the safe choice abd with the motor's health riding on the choice I'd go with it.