Change Timing Belt Based on Miles, or Time?

I have a 1985 Corolla. I use public transit to commute, so I only put about

3,000 miles/year on the car. The timing belt was last changed in 1994 at 58k miles. Nine years later, it now has 104k miles. Should I go ahead and change it now since so much time has passed, or is the wear based only on miles, which would mean that I should change it at 120, which would mean years from now?

I guess I am concerned about dry rot, exposure to the heat/cold cycles, etc. BTW, my belts and hoses are in good shape.