Permits are good for 180 days from the date of issuance, or from the date of the last inspection.  In other words, you need to be ready for your first inspection within six months of receiving your permit, and you should be ready for another inspection every six months after that (or sooner, of course!).
If you find your project delayed for unexpected reasons, you can apply for an extension.  This doesn’t increase the cost of your permit, but the Building Official does have discretion in granting extensions – so you’ll probably need a better reason than “I just didn’t get around to it.”