"How long do you expect UPVC windows to last". Well that's a very difficult question, because it seems by the cold caller's we get, its about every two weeks :O)
But going back on the subject of the 'gearbox', I have changed the one on the rear door. Checking the main dealer network, I found the replacements far easier and cheaper on eBay. The same applied to the window handle and lock devices. Change of door locks from Toolstation.
The best part of the above was the ease of fitting, usually by screwdriver.But watch out for door locking fitting, the old holes might require attention, because of previous 'slipshod' work on original assembly.