As a person with long hair, I'm here to tell you that it grows quicker than you think. That's a year and half, two years at most. Maybe less depending on how fast a person's hair grows.

I think the length of her hair had to be drastic enough of a difference between the two movies in order for us to be able to tell the difference in time during the flashbacks. Probably her hair would not have grown that much over that period of time, but I think it was used as an indicator to show that when she was in the room with Adam was before the events of the movie!

Timeline goes like this:
(1) Saw I-Amanda has long hair at the police station interrogation flashback scene
(2) Saw II-In the flashback showing John convincing Amanda to join his cause, this takes place right after the police station scene; shortly thereafter, Amanda cuts her hair
(3) Saw II (or III, can't remember)-Flashback of Amanda with short hair attacking Adam
(4) Saw III-Flashback of Amanda with short hair bringing Adam to John's trap set up for Gordon and Adam
(5) Saw II-For the entire movie in the present-day scenes, Amanda still has short hair after having helped Adam trap Gordon and Adam for the events of Saw I; now John is helping Amanda set the trap for Det. Matthews
(6) Saw III-The beginning of Saw III takes place some months after the events of Saw II (can't recall how many), enough time for Amanda to grow her hair out

If there's a 6 month time jump, it's possible. Although I think the actress was wearing extensions, anyway.

Six years growth? Think that might be a bit of an exaggeration, but to be fair there is a six month time gap between Saw 2 and 3.

You realize women get extensions all the time?

Saw II *