I smoke 'em til they don't taste good anymore, and sometimes even longer than THAT. I smoked a Choix Supreme yesterday way down until it damn near burnt my lips.
If you are smoking properly aged cigars then the flavor will get better and more intense as the cigar gets smaller. I normally nub every cigar I smoke because they taste so good. If you are getting a bitter taste at the 2/3 or 3/4 mark you might want to drop your humidity and age your cigars longer.
I usually smoke to about an inch. I think it was Churchill that said the second half of a good cigar is only as good as a common drugstore cigar but I will contest that anytime. I find this is even less true for full maduros for some reason.