Wolfenstein

Easiest way to get this achievement, upgrade your Veil ability with the first 3 upgrades from the top, stand on a veil pool, turn on the Veil ability (Down on D-Pad), and let it go.

Your energy is not draining, but you stay in the Veil. Move the controller every so often, and in some time (depending on how long you've played), you will have the achievement.

All I did was go to an area with a veil pool and no enemies and sit there in veil mode. If you have a charge pack on your controller and plug it into the 360, it wont go idle so you can just go about your business for 2 hours without having to move the cotroller any.

In the Dig Site mission you collect the Thule medallion, the artifact that allows you to use the special powers in the game. Directly after acquiring it, the Russian guy from the Golden Dawn will give you a blue crystal during a cutscene which unlocks your ability to enter the veil. In this room there are a pair of small alcoves with some blue stuff coming out of the ground (called "veil pools") that have an infinite supply. Simply hit down on the D-pad to enter veil mode, stand on top of one of these pools, and walk away for two hours; as long as you see the weird little floating bugs and an endless ticker tape parade, you are in veil mode. Just make sure your controller is wired (or connected with a play and charge cable) otherwise it'll shut off and cause the game to pause itself.

It is also worth noting that this does not accumulate between playthroughs. I had a savefile from when I first tried this game back in 2009, then started another playthrough on the date of this solution to not die/get all collectibles/etc. I had to wait a full 2 hours in this new campaign game to receive the achievement.

You can also just play through the game with Veil activated. Their are so many veil pools that by the time you run out you should be near another.