Afaict it's functionally almost equivalent to clicking "Play Now" on QM with only the Arena's forced Template enabled, although of course with Idle now rewarding you when you win.
Given the templates and QM settings in general, the time investment is small. It may be higher if your QM rating is high since you will be more likely paired with a serious opponent (although I don't know how well this matchmaking works- my opponents so far have all been similar to the opponents I'd get on RT QM). So far, about 67% of the way through Ursa:Luna, my games have been:
1. A Small Earth Auto Dist arena (https://www.warzone.com/MultiPlayer?GameID=23283000
) that took ~15min vs. a ~480-rated opponent (I was rated ~80)
2. A Small Earth 1 Wasteland arena (https://www.warzone.com/MultiPlayer?GameID=23295274
) that took ~9min vs. a ~480-rated opponent (I was rated ~490)
3. A Strategic 1v1 arena (https://www.warzone.com/MultiPlayer?GameID=23329112
) that took ~5min vs. a ~400-rated opponent (I was rated ~350)
4. A MME Commanders LD No Cards arena (https://www.warzone.com/MultiPlayer?GameID=23357009
) that took ~2min vs. a ~125-rated opponent (I was rated ~70); this game ended fast when my opponent surrendered upon realizing what template we were on
Another angle on the time investment is that you're probably not going to be able to do much more than wait if you spend the same time on WZI. Imo Arena complements WZI very well since you play a short game (or games, if your QM rating is high and you end up playing good opponents, or if you lose for some other reason) and then you come back and you have these extra armies and stuff to do in WZI. Most of my arena games have been the perfect duration for having a new Draft when I get back.
Imo SEAD is a bad fit for Arena since you could end up spending an inordinate amount of time on it if your starts aren't great relative to your opponent's. SEAD and SE1W also have template specialists that are different from the main strat community, so this could be an unpleasant experience for a good strat player used to normally winning their games against randos pretty fast. I've been on the other side of this, as a QMer playing SEAD who got matched with someone playing Idle, and that person who got matched with me was not having a good time with the Arena on that template. They had to play at least 3 times for their Arena bonus and had to navigate the auto dist lottery of SEAD while also getting paired with good opponents they couldn't rely on to make mistakes if they had superior starts.
Edited 7/28/2020 17:55:21