If you're a tank using Clench over Inner Fire/Beast, then it's probably because your group wants you to run Minor Brittle rather than Major Maim specifially, or you're simply looking more towards supporting your random PUG dungeon more than simply taunting and blocking, in which case keeping Brittle up is more important than the Maim, but Major Maim will occur naturally and constantly as a result.I wouldn't bother to keep Major Maim up all the time, to be honest.
Minor Maim has a few sources that allow for a good uptime - Talons (if on DK) and Heroic Slash are the main ones. That's worth paying attention to.
Major Maim should be an option for very specific attacks. If you know that the boss has a particularly high-damage heavy attack and see that spinning up - then go for it. Or if a boss is about to do a mechanic that hits many group members really hard (Warrior's Starfall in HRC, for instance) - that's a useful moment for this debuff.
Trying to keep it up all the time is way overkill, though.