Sparring isn't "technically" an endurance activity compared to what people (at least me) usually think of as endurance sports - marathon/triathlon, cycling, etc. It's in the same vein as most fight sports where you have some recovery interspersed throughout, but I get what you're saying. I always like to have any fighters I work with do intervals that are about 20-30 seconds longer than what they need - if they have 2 minute rounds, then have them do intervals at about 2 1/2 minutes per burst so that by the time they get around to a fight it's a cakewalk in terms of endurance.