Two nights a week is all I can devote and we've been pretty successful. I can't invest any more time than that because I also like to have some sort of life outside of the game and work.
Plus, logistically, organizing people who will attend more than two nights a week is a nightmare. I'm very up front with everyone about things like this though. In each meeting I remind everyone at least three times that "We are not a hardcore guild" because it's practically our slogan and it needs to be repeated because, for whatever reason, people join, raid with us, and think after six months we'll magically grow 10 more players and immediately dive into Mythic raiding even when such a thing was never, ever mentioned, breathed or alluded to. Thankfully, the core members of my group who have been with me for years know this and also want it. It's a good system.
But for real: if someone wants us to change our shit and drop heroic for mythic they can gtfo and find another guild. That's not my shit and it's part of the reason I started this guild with the original GM. There's plenty of hardcore experiences out there. I have no interest in being one of them. We've existed for ten years so I'm doing something right, lol.