It really depends what level of performance you expect. You could run them 'til they're bald for 20-25K miles, but wet handling would suck and you'd risk being in an accident from sliding all over the place. Dry grip would suffer too, but not as bad.
If you want the highest level of tire performance you should get new tires when your current tires are 1/2 to 2/3 worn -- which would be now.
It really all depends on your budget and your tolerance for less than stellar handling AND braking.