That depends. Are you planning to let it sit in your detailing cabinet, or use it? If you're going to just admire it on the shelf, it sounds like you could save it for a very long time. But I bet you plan to use it. :idea In that case, you'll discard it when it has encased a bunch of contaminants.
Personally, I've adopted BradB's approach. When I clay, I cut off thin slices from the bar, then I flatten the slice down into a little piece about 1/4"thick and the diameter of a silver dollar. I use that slice til it seems to have some grittiness, then I toss it. Other folks will talk about folding and kneading a bar to re-use it. But personally, I look at it this way: I'm cleaning and being steward to a car worth many thousands of dollars; so I'm not going to "push the envelope" to economize on a $10 clay bar.
I'm sure that other Autopians will weigh in on this when they wake up tomrrow.