May be the measure should be how harmful it is to continue?
For an MMO, you're engaged socially (presumably in a guild, rather than soloing). In a mobile grind game, you spend hours constantly checking on progress for that endorphine hit.
Same as with any addictive thing, you can use and you can abuse. And the change isn't instant.
You can play MMO socially in a guild, or you can simply grind max levels / rare items endlessly.
You can play an IAP and ad-riddled mobile game to kill some time when waiting in queues, or you can have it become a part of your life checking the game everytime you have a minute of downtime.
And the transitions between those states happen impreceptibly, and there is no defined line beyond which you may consider the activity harmful.
For an MMO, you're engaged socially (presumably in a guild, rather than soloing). In a mobile grind game, you spend hours constantly checking on progress for that endorphine hit.