Both the MKI and MKII are non-interference engines. This means that if the timing belt breaks, no damage will directly occur to the engine. Obviously you can't drive it with no timing belt!
The timing belt should be changed every 60,000 miles. Changing the timing belt is a tough job, due to the lack of clearance between the front of the engine and the side of the car.
When you change the belt, change the idler pulley as well, as it's probably on it's way out.