I'm speaking of road tires here, 700-800 mile old Michelin Pro Race 2's specifically.

I know that cuts and small holes are to be expected, but when is it time for a new tire? I've got a (central) gash about 1/3 the diameter of a pencil eraser on my rear tire. I've used several applications Loctight rubber sealant that have not held entirely. I haven't flated out yet but I am feeling less confident on the long rides.