I completed three playthroughs.
1 on Balanced Mode as Tav
1 on Balanced Mode as Durge
1 on Tactician Mode as Origin Astarion
I did not change the difficulty at any time, and have completed the vast majority of every battle I could find.
Act 2 is the shortest, and Act 1 and Act 3 were pretty equivalent. Percentage of time I spent per Act:
Act 1: 35-40%
Act 2: 20-25%
Act 3: 35-40%
If I were to guess how long it would take you in terms of hours, probably the higher end of this range:
Act 1: 30-40 hours
Act 2: 20-30 hours
Act 3: 30-40 hours
I felt Act 3's main map was more dense than the maps in Act 1. There was more to do, per square foot traveled.
Act 2 can be long or short, depending on if the person chooses to talk their way out of battle or do the battles
Act 3 can also be longer or shorter, depending on how many companion side quests you're doing. There were many more optional battles, however you had to do combat for most of them
