There are two methods to earn this achievement a legitimate method and an exploit
Purchase Possession 1 & Find a small rat vent in the base of a wall.
Possess a rat.
Run into the vent and stay there for 3 minutes. (You will never be kicked out of the rat while you are in the vent).
The achievement only unlocks when you re-enter your own body, so don't stay in the vent for 10 minutes waiting for it to pop.
After purchasing the Possession power, find a swarm of rats or a crowd of people (possession 2 required). Boyles Party is great.
Make sure you have at least 6 - 7 Mana Vials (equip bone charms which aid in mana regen/consumption etc). Make a save before you begin.
Cast possession and wait to be kicked out.
When you return to your body, Immediatly re-cast posession & enter another body. Rats & Fish die after possession which is why you need a swarm. Humans start vomitting and take time to recover.
Keep possessing bodies. You will use loads of Mana so make sure you have plenty of mana and plenty of bodies.
When you reach the 3 minute mark the achievment will pop once you return to your own body.
Reload your save to get your mana back.