Charm, viable when?

[Ropethunder]

I tried this the other day when my enchanter reached level 12. With 201 charisma, tashing first, the first dark blue I charmed held for 5 minutes.

After that it seemed completely random of charms breaking < 1 minute, ~1 minute, 2 minutes, a few seconds (on greens). It was so ridiculously inconsistent that I just gave up.

Very disappointing.