The best timing to trigger this quest is right after you've unlocked velen's notice boards, look for a notice about sir geralt it's got this fancy bridge crest on it. Once you grab that notice make your way southwest of oxenfurt, to stonecutters' settlement where some toussaint knights are dealing with bandits. The exact positioning here matters, since you'll want to approach from the main road to trigger the dialogue naturally and you'll have options for how to handle the situation. If you're going aggressive, stay close to the knights during combat since the bandits love to flank, but if you're talking it out keep a bit of distance so you can read their body language.

Good to know you can also jump straight into this by picking blood and wine from the main menu which sets you up with level 34 gear, though you'll miss some context from the main story. The knights' attack patterns during the bandit fight are pretty straightforward they'll usually telegraph their strikes with a slight pause, so time your dodges about half a second after you see them shift their weight. They're actually decent allies if you help them out.

Here's something interesting the whole sequence flows a lot smoother if you approach during daytime, since the lighting makes it easier to spot the settlement's landmarks and catch those combat tells if things go south.