

She’s evil, he thought, as he climbed a staircase to the seventh floor, she’s an evil, twisted, mad old –" "Harry had never before considered the possibility that there might be another teacher in the world he hated more than Snape, but as he walked back towards Gryffindor Tower he had to admit he had found a strong contender. Later in the series we get get a few small retcons:

Now, even if there were more than just this one staircase (which isn't implied), this means that it's definitely below floor 3, in the first book. When going to the trophy room to duel Malfoy, they went up a staircase to the third floor. From this quote, it's possible for the entrance to be more than two floors up, but I think it's implied to be on the second floor, and it's definitely not lower than the second floor. Then a brief conversation is described and they reach the common room. When going back to the common room after defeating the troll in the girls' toilet (which is on the ground floor), Ron and Harry don't talk until they've climbed at least two sets of stairs. When Percy first leads them to the common room from the Sorting Ceremony's feast, they climb multiple staircases - that puts it on a floor higher than floor 1. The first half of the series it seems to either be located on one of the lower flowers and the later books place it much high n the castle. It is never exactly stated where it is located in the books, the 7th floor thing was an invention of the films and video games that stuck.
