Envenom, utori pox, uses 30 mana and has 40 seconds cooldown. That makes it not useful during hunts, however it's very useful to make damage over time at bosses. Since it's earth damage, it would be very good at the bosses that have it as weakness or are neutral to it. It's basically the best spell for druids to make damage over time, right next to the Soulfire rune, if you want to push your damage at boss.
Keep in mind, that it would be only lasting for each stage of boss - example: Oberon has more than one stage and you would need to use utori pox on each of them.
In my opinion, it's very useful spell at bosses.
Examples where to use it:
Alptramun at Dream Courts
King Zelos, final boss in Grave Danger
Ferumbras Mortal Shell