I guess the best way I can describe Act 2 without spoiling it is that part of Act 2 has Geralt calling in some favours from people in Velen, Novigrad, and Skellige. And so the time taken to do this would be dependant on how much you've done before triggering the Act 2 arc. I think like me a lot of people like to entrench themselves in the side quests and subplots involving major characters, even when these happen after technically completing the Act 1 "Search for Ciri" portion. Best example I can give is Triss in Novigrad; she has her own arc but you don't technically need to finish it entirely before getting the necessary information to complete the "Search for Ciri" objective in this location. So when Act 2 comes and Geralt wants a favour from her, some people would have already done enough with her for that to meet a conclusion, and some would have left her quest unresolved and thus need to go into it before getting to the favour.
For example, I didn't have to do anything in Velen and Novigrad to call in my favours because I'd already completed a shit ton of side quests related to these characters. Skellige though I had some stuff unresolved, and thus wanted to finish these arcs before asking for assistance.
That part of Act 2 will vary in length depending on what you've done at your own pacing prior.
