According to workshop manual timing belt should be replaced every 100,000 miles under normal load or 75,000 under heavy load. Normally, you need to replace tensioner and pulleys with it. Good mechanics also recommend to change water pump at the same time. I replaced mine at 60.000 because I was not sure about the full history of my VX.
2001 Isuzu VehiCROSS (build in progress): 4JX1/A340/part time, 2" body lift, front Aisin hubs, rear Aussie Locker, 4.56 gears, OEM rear sway bar disconnect, AirFlow snorkel, cabin air filter
1980 Jeep Cherokee (assembling): 1.5" body lift, TBI, HEI
1987 AMC Eagle: stock