The mileage/time interval for gearbox oil changes given by Talkingcars is correct.
The change interval for the gearbox oil was extended to 105000 miles when Rover started using MTF94 for factory fill of the PG1. MTF94 is very long life formula oil - no need for frequent changes, and made the Rover gearboxes effectively 'fill for life' (there being relatively few that would reach 107 thousand miles
Previously, on standard life oils, the recommended change interval was something like 30 or 40 thousand miles.
If you drain the oil from a PG1 filled with MTF94, and use a standard 75W80 to refill, you would need to reduce the time before the next change accordingly. There should be no problem with using MTF94 for 100 thousand miles (provided it is checked and topped up if necessary - lack of oil is the biggest killer of gearboxes rather than lack of oil changes), but obviously it will do no harm to change more frequently.
I changed the oil in my ZR soon after I got it (around 40K), but as the oil that was drained out was still fairly clean and clear, I think I wasted my time and money.
As an aside, the IB5 box fitted to K series engines of 1.6 litres and below from July 2003 had no dedicated drain plug, and was designed to be filled for life (regardless of mileage) with only topping up required when necessary.