The simple answer to this question is smoke the cigar as long as you like! Handmade, premium cigars can be smoked to the very end if you wish. Do so with caution however, as flavour change as you approach the head is to be expected,
Moisture and tars accumulate near the head of a cigar as you puff. At some moment during the smoke, typically after the final third of the cigar, the flavor of the cigar can change, and get bitter, or somewhat harsh. That moment is referred to when a cigar ‘turns.’ The oils and moisture have accumulated and have tainted the flavor of the cigar.
Purging the cigar at this stage may help improve the profile and extend the smoking time, but if not, it’s best to call it a day. However, there is no set rule for when a cigar does or does not turn, and many cigars may show no off flavours when smoked to a nub.