It depends. Holding tension is not the problem. Care for it well and it lasts many, many hours!
I once broke some VS in several hours with poly crosses. But that was unusual.
My standard setup of VS mains and poly crosses usually results in heavily notched gut after just 2-4 hours. Moving a poly cross out of and back into position results in a load "snap" as the poly snaps back into position. But the gut mains remain free to slide over the poly crosses so it doesn't seem to effect performance at all. In fact, no matter how hard I hit and no matter how much spin I apply my string bed seems neat and tidy with a gut/poly hybrid. With full bed mutli it's a mess after a few hours (when I demo'd the PSLGT with NXT17 I was the first to use that particular demo frame...after four hours the strings were a mess.)
For me the poly begins to go dead before the gut breaks, but I'm certain heavier hitters can bust through gut in just a few hours.
But as for tension loss it just hasn't been an issue for me and I play 2-3 times per week at 1.5 to 3 hours per session.