If we're bottoming out of mana in 7 minutes, it won't make a difference if you blow Assize on cd or not. Your overall mana income over the fight is the same. You'll empty in 7+ minutes regardless.

You'll still get to use it as a heal cooldown until you approach that 7 minute end point. Assize having charges will not make the problem worse, you don't lose any mana, but it will make small QoL improvements.

WHM of course has a whole range of other issues, including mana economy, but that's another discussion.