The honest answer is that it depends on where the cigar came from. A cigar bought from a serious local shop with its own walk-in humidor may need only a day or two to forget the journey home. The shop has done the patient work for you. A cigar shipped to you from an online retailer wants weeks rather than hours, regardless of how reputable the shipper is. Shipping is hard on tobacco, and the moisture gradient across the wrapper, binder, and filler will not have settled in the few hours since the box arrived on your doorstep.
The reliable test is your nose and your fingers. Hold an unlit cigar an inch from your face, breathe in slowly, and pay attention. A clean, sweet, woody, leathery, or barnyard nose is a cigar ready to smoke. A sharp note that pulls your nostrils back is a cigar that wants more time. Squeeze the body gently. Lightly springy is good. Crunchy or spongy is not.
When you are unsure, the cheaper test is to wait. Cigars are patient with us. The least we can do is return the favour.
For the full case for waiting and the chemistry of why early cigars burn unevenly, see How to Rest a Cigar.