Having had personal experience of the disastrous and very expensive aftermath of having a timing belt snap, it's worth bearing in mind the following two points:
1) Hard driving shortens the life of a timing belt. (Jeep, for instance quote two recommended mileage intervals for their engine's timing belt changes - normal and severe.) Taxi driving, with all the stop start driving which puts extra strain on the belt, I think anyone would agree, sits squarely in the severe category. Jeep quote a timing belt change mileage that is 25% less than the normal mileage change, for this type of driving.
2) As has been mentioned in other threads, the water pump should also be changed at the same time as the timing belt. This is recommended even in some of the most general vehicle maintenance manuals as being just good practice. It also makes economic sense because most of the labour costs have already been incurred by the changing of the timing belt. A water pump seizing up can be just as damaging to the engine as the timing belt going, and a leaking water pump (which sometimes happens to the old one soon after a belt change - I've had that happen as well) can also be avoided by changing it.
And I fully agree with whoever said that the engine never feels the same again, even after all those very expensive repairs. It's far better to do the preventative maintenance in the first place.