They could do no timer easily by making a wipe kill your key. They could disable the release button for the whole dungeon like a raid encounter, and only make it available when the whole group is dead. This would allow for resurrections if at least one person survives if they have the ability to rez (it would also give shaman, paladin, monk, druid and priest dps extra utility). Once the group wipes, it would result in the equivalent of the time expiring.
I'd much rather M+ feel like a dungeon crawl rather than a nascar diablo race.