I'm not sure exactly when you should do yours, probably before 100k. If yours breaks, you'll be stranded on the road side and it could be dangerous because you'll lose power steering and brakes, but you won't destroy your engine. On an SVT, the valves retract as the piston moves up, if the timing belt breaks, the valves and pistons will collide, usually destroying the engine, though sometimes the damage can be repaired, but it would still be very expensive.
When you change the belt, you should change some other components with it, like idler and tensioner, you'll have to research exactly what's recommended for your motor, maybe water pump too.
The belt on my wife's PT Snoozer broke because the tensioner seized up, that was scary. She almost crashed because of it, thank goodness she didn't.