l have 66,000 miles on the clock.

that depends on when it was last replaced. 30,000 miles is plenty.

usally around 150 to 200,000 miles

start with a fresh one now, then once every two years.
parts and labor may seem some what high to you, and l would agree, BUT, it is cheeper that the cost to repair the engine damage, when it finally breaks.

most timing belts need to be replaced at or before 60,000 miles.

most cars 60,000miles better safe than sorry do it now bude