Judging by what I've read in Famine's Endurance companion & on my own experience, the chassis deterioration is mainly dictated by mileage. However it's hard to know whether driving purely on grass/sand, in GT4, will accelerate chassis wear. But I don't see why not.
Also, as a side note, putting the rollcage on Group C's/LMP's/Etc. is not advisable. These cars are already equipped with a rollcage and the suspension is set for their current state of stiffness. Throwing the 'Increase Rigidity' on only makes the chassis exceed the bounds of their suspension, generally making them harder to drive as the suspension can no longer match the chassis accurately.
FormulaGT
Also, as a side note, putting the rollcage on Group C's/LMP's/Etc. is not advisable. These cars are already equipped with a rollcage and the suspension is set for their current state of stiffness. Throwing the 'Increase Rigidity' on only makes the chassis exceed the bounds of their suspension, generally making them harder to drive as the suspension can no longer match the chassis accurately.
FormulaGT