Good words so far. It really depends on what kind of driving you do and how much tread @ the time of year. I ran leagally bald tires all summer on one of my cars but I didn't drive it much in the rain and only put about 5k miles on it since Spring. Next summer when my KDW's are legally bald I'll run them as well as I'll have new tires for the other car for when it rains.
If you do a lot of hihway driving in the rain, get new ones sooner. The first time you hydroplane and understeer into a curb, you'll know that it's time to call Luke.
