It will depend on if you use them only at the track or if you drive on the street a lot with them. Also the longer burnouts will wear them out fast. When it's hot out and good prep at the track, you don't need as long of a burnout. When it's cold and the track is cold is when you may need a longer burnout. Try to do no more of a burnout than is necessary. This will depend on your power level also. The more power you have, the more of a burnout that is needed possibly.
But I normally get 1 > 1.5 years out of mine, depending on how many times I get to the track.