I think I usually take around an hour and 30 minutes

Ive seen some people say itll take 20 hours, is that to unlock all the units and stuff?

The first few runs are about 30-40 minutes. Then they start getting to be an hour long or longer as you get more and more difficult dungeons to complete. It runs long for this kind of game, but it saves at the start of every floor, so you can take breaks.

20 hours... I think it'd be hard to beat ALL the dungeons in that time, but maybe most players see the credits roll for both characters by then?

The game is separated into individual runs. You'll start over with a new team at the start of every run, with each run taking maybe 1-2 hours, depending on how fast you play. Also, you gain a special currency through playing which can unlock additional features/new units that you'll see in future runs. Maybe it takes 20 hours to unlock all of these, I don't completely remember but that estimate seems about right.
You'll also unlock (optional) higher difficulties every time you win. You're probably a god gamer if you get through that in 20 hours.

After beating the witch's maze and game credits show up, there are three new unlocked dungeons. Does beating them offer any more ending or cutscene or not?