I believe it was soon after Guts joined the hawks. Not that he gained so much experience in such a short time or gained more strength but simply because we're talking about Guts here. Griffith merely surprised Guts with his actual skill and techniques. It was rare for Guts to encounter a human that could hold out against him even at that age.
One cannot deny his almost inhuman talent and reflexes during combat. He already surpassed Griffith in terms of strength, endurance and stamina. Heck! You might even say that Guts is "engineered" to be one of the strongest humans ever when seeing his performance in battle.
The only advantage Griffith had over him was his technique to counter and deflect even the hardest blows. Guts sure was shocked but he immediately analysed this technique during the battle itself and already tried thinking of a way to defeat Griffith. Guts sure is adaptable. A genius in his own way in my opinion.
I must say that the chance of him beating Griffith was rather great in case he weren't wounded. However could argue that we never saw Griffith at his best during that fight and that he was merely toying a bit and only got serious at the end. Knowing the calm and calculating Griffith I don't think he would've even started a fight with Guts unless he was sure of the odds.
This being said. Griffith knows how to spot talent.
Guts probably would've won if there was a rematch some weeks after their duel since no human can hold out in battle against the likes of Guts. The only difference is that it would take Guts a bit longer depending on the strength of his human opponent.
(Though there were humans stronger than Guts at that time like Boscogn but Griffith wasn't one of those humans... In terms of battle of course)
That being said... It doesn't take long for Guts to see and know how his opponent fights. He's very quick at learning this and he already fought Griffith so Guts would have the advantage.
