What lvl of stealth do you get this? How reliable is it?

Detect Hidden is received on your 16th train in Stealth. I believe your Stealth, your level, and the levels of both possessed by the enemy are all factored into how close you must be in order to spot your victim. Kehvrynne Quickblade, Professional Damsel of Distress
No matter how subtle the wizard, a dirk between the shoulder blades will seriously cramp his style.

No, the level of the detecter is compared to the stealth spec of the detectee to determine the "detect radius" of the detecter.

The stealth of the detecter and the level of the detectee are not considered.

To oversimplify: Detect Hidden doubles the "detect radius" of a given detecter.