You should be good for many many many...many more hours. There should be no need for you to change out all your teeth. I have changed a few because of very rocky conditions but it isn't normally necessary in normal soil conditions. I would grind away if I were you and not think twice about grinding dirt deep down.
I have gauged up some pretty hard granite before the teeth gave way, and only replaced them when absolutely necessary. One time I replaced an entire tooth assembly simply because I rattled a tooth loose from all the ledge I was grinding through, do check your torque once in a while and you should be fine.
It's never a bad idea
To have a few extra teeth on hand but if you don't have very rocky conditions then I can't see why you'd need to
Change them out for a very long time.