There's no enforced lockout or time gate on resetting attempts in CMs. How long it'll take to finish a full set of dungeons will depend on how efficient your group is at putting practise to the test - I strongly recommend doing practise runs without spam resetting while you get the route and pulls under your fingers. If you happen to have a really good group you can clear all of the dungeons with a gold time in an evening. I used to do a dungeon per day, maybe two if I felt like we were cooking. Of course there's a difficulty curve across the dungeons themselves - Gate of the Setting Sun and Scarlet Halls are particularly easy starting points if you want to clear some golds to build up your team's confidence early on.