I wouldn't get caught up on the acts too much. Act two and three will fly by in comparison. Without spoiling it's basically structured like:
- Prologue: White Orchard
- Act 1: Three "Search for Ciri" objectives aimed at introducing you to Velen, Novigrad, and Skellige and key characters found within. Varying lengths for each of these quests (Skellige variant with Yen is quite short).
- Act 2: Core objective with optional related objectives that relate to characters met in Act 1 culminating with an obvious climax.
- Act 3: Final stretch of mandatory objectives that can be tackled in any order culminating with the endgame climax.
As said, most people will spend majority of their game time in Act 1 as being introduced to the three regions goes hand in hand with doing a ton of side quests as well. Additionally, the part in Act 2 related to characters ties into side quests that you may have already completed come Act 1. Act 1 mandatory stuff feels by far the longest, supplemented by all the optional additional work you'll likely do. Act 2 varies in length depending on how much you want to do, and what you already did in Act 1. Act 3 is more focused and easy to flow through.
The game never specifies acts, mind you. But yeah. You will
probably do most of your playing in Act 1. But depending on what you do or want to do Act 2 and 3 can still go on for some time. I mean, it did take me ~144 hours to do basically everything.