This settles it. Commentator of the match between Wawrinka vs Donskoy in St. Petersberg open today stated that he was told by most of the well known manufaturers of Polyester strings told him that max lifetime of poly string is 45 minutes. That is big part of the reason why ATP tour pros change rackets during the match. I say Part of the reason, because another big reason is Tension loss as well. So yea, all those sensitive guys out there thinking why does my poly string feel dead after only 4-5 hours of play,, You are not imagining it. It really is going dead. So if you can afford it, string polys often.