It comes down to the type of carpet as well and the underlay.
Woollen carpets bounce back faster than nylon carpets but even then it takes a couple years.
I had a 3ft tank (3 x 1.5 x1.5 ft) only on 2 edges of 18mm custom wood and that left an imprint in my carpet for 2 years.. the tank was in the same place for 2 years. Now you can't see it.
But I agree with the others before me... Best if you can spread the weight of the tank as much as you can. Placing a board of customwood or Ply underneath a tank will definitely help protect the carpet.