When in doubt, read what you quote
A Scroll of Detection gives a one-time check for secrets at 75% change with a range of 10 meters. So once you use it, where ever you are standing is the center of the range, and any secret door, trap or Invisible Swamp Donkey* within the range gets one, and only one, check to be discovered at a base rate of 75%, in-game mechanics such as level difference between player and the secret item may reduce or raise that chance. For clarity, there's no duration. It's a single, instantaneous check.
Also, if a player uses a Scroll of Detection, in-game text in the combat log will inform the player if there's any secrets left undiscovered in the area. If no secrets are discovered by the scroll and no secrets are left hidden on the map, no special text.
http://www.exiledkingdoms.com/wiki/index.php?title=
*Due to various rulings by the Thuram wizard high court, it's unclear if the Invisible Swamp Donkey can be located this way (and please report it with a screen shot if you do find Invisible Swamp Donkey).
