Again it’s two that you actually apply. One every 21 seconds, and another every 45 seconds.
Over the course of two minutes assuming no haste, thats 7 (5 for Rend/2 for Thunderous) GCDs out of 40 or 17%of your GCDs commited to applying Bleeds. That doesn’t account for things like Blood and Thunder, Bloodletting, and SkS but the point is you’re not constantly refreshing bleeds, nor is it a major part of your damage profile if you maintain good uptime.