I just bought a 2001 isuzu rodeo has 146.000 miles on it how long before i have to change my timming belt?

I couldn't tell you specifically for this vehicle, but generally, they are replaced at 90,000. Are you certain it wasn't? Be careful; if this has a valve design such as (some) Hondas', it could damage several (all??) valves if it breaks. You can't beat calling the dealers' service center and asking about maintenance schedules. I guess I'm uming you don't have the owner's manual??