For a 10-seconds long rewind functionality, only 64MB RAM is needed (128KB * 50 frames * 10 seconds)
(But way more efficient methods certainly exist as the GBA emulator [https://youtu.be/Sfc_1NKbiKg SkyEmu demonstrated super long rewind] while needing very few RAM)
To reverse the game at 1x speed, users press and hold the rewind button. And at any point, releasing the button makes time goes forward again.