How much mana does Premonition use when it triggers?

[Orion]

And what about other sustains that give active defenses on a cooldown?

[Theyleon]

It uses none.
You already have to pay the max mana cost for sustaining it, there is no additional cost.

[Orion]

Several times, I have taken Premonition as an escort reward on a nonspellcasting character, thus revealing a previously unused mana bar. I would turn on the sustain and have something like "68/68" mana left, or whatever it was based on level and willpower. However, as I clear dungeons, something gradually empties my mana bar until I have 0/68 left. What is that?

[jaumito]

There are talents and items that monsters can use to drain your mana. Just equip an item that replenishes mana over time and rest when you notice your mana bar is empty.

[cctobias]

Several times, I have taken Premonition as an escort reward on a nonspellcasting character, thus revealing a previously unused mana bar. I would turn on the sustain and have something like "68/68" mana left, or whatever it was based on level and willpower. However, as I clear dungeons, something gradually empties my mana bar until I have 0/68 left. What is that?

Yes I have had this happen as well and at first I thought maybe it was premonition. I was considering this because I was thinking usually if I lose mana its because of something manaclash you lose an obviously large amount.

But really its rather easy to lose mana in 20 point chunks and not realize it simply by running into something with arcane resource burn since that would just show as 20 points of arcane damage on their melee attacks and be hard to even distinguish from normal arcane damage.

If you are killing a lot of things especially in melee and you have no mana regen at all it basically will just get worn down. its just that as a normal mana class with even modest regen you don't really notice a losing 10 or 20 here or there. As that regens during auotexplore. Assuming you are playing normal and using autoexplore etc.