hawlyns711
.........and you can get away with this in New Hampshire?
Here in Wisconsin, that has "ticket" written all over it. Although I've been known to drive a vehicle with a malfunctioning speedometer from time to time, but at least they had one.
Rule of thumb in your case, every other month I think would be plenty. Spring and fall would prolly even be enough. If you could give a guesstimate of the distance to work and back, that might influence the answer.
It's a bit like changing the oil on a mower; when do you change it? Simple, when it starts to look dark brown, and not the light brown stuff you poured in.