another tire question

ive been running the hoosier 34.5x17x16 dbl beadlock wheels. In top sportsman ive been running mid 6.80s in good air and slower. 60' times are generally 1.05 to 1.07 area. my question is how many runs are guys putting on there tires and how do you determine when to put new ones on? my set I ended last year with has about 25 laps on them. I am also switching to a 4.29 gear from a 4.11 so the hit is gonna be a bit harder as well.