If you're an amazingly keen observer, tactician and strategist, in a normal game you can PREDICT with 80% accuracy the winner by about Round 7 if Barbarossa, Round 9 if Sealion. Of course, it depends on dice/the situation, but I think that's the middle ground. For normal people, it's probably Round 10/11.
I'd say that 20% margin of error is worth playing out. Only throw in the towel when there is no path to victory whatsoever. More than one victory has been snatched from the jaws of defeat through an incredible blunder by one's opponent or an incredibly lucky major battle - and those are the games that are remembered.