Depends on what you get, how you drive and where you drive. The Toyo 888's we run have lasted over a year. Roughly and without thinking too much it would be about 4 days Collie, 3 Barbagallo, 4 or 5 half days on the long track at RAC, couple of Autotests, hillclimb, speed dome and a few PDC's.
If you look after them and dont leave them on the car they'll last pretty long. And the Toyo's can be spun on the rim which helps get some extra life.