its mostly up to opinion...

you can smoke a cigar all the way to a little nub or you can smoke it so a 1/4 or 1/3 is left...
some cigars will start being less good towards the end...
i just smoke the cigar still it stops being as good...

again the puff rate varies... some say 1 a minute or so... I personally usually average 1-3 puffs a minute varying on how im feeling how much time i got etc...

but im also still pretty new... only been smoking for 3ish months...