Depending on what you set the difficulty to before starting, it depends on how fast you'll get there. On the average monster setting, It can take me anywhere from 30 mins to an hour to complete a part of Act 4. The map and number of mods can slow you down if you get a bad map rng and have to trek across the map.
On Average I'd say the ends of Act 3 and Act 4, depending on Solo or party play, can take anywhere from 3 to 7 hours to complete.
On a low difficulty it would take both my brother and I at least 3 hours to speed through the second half of Act 3 and Act 4 if we speed past low mobs and kill only when overwhelmed which rarely happens.
Oh...Lower difficulty also = crappy weapons and item drops. If you just wanna finish, go for the lowest difficulty you can, but you won't get any good drops, and I think a few Set recipies drop in Act 4 at higher rates with increased difficulty.
Just thought of a comical answer to your question...doesn't really help but its a very accurate description. Diablo is just a very bad itch on the back of the world just out of reach of being able to scratch it.
