How long does it usually take for you to prepare for an AC?
"It depends" (sorry!). If it's your first or among the first, and/or you've applied for a lot of firms so you need to brush up on your research, then give it a few days of prepping with a few hours each day to go through your CV, think through competencies, commercial awareness, etc.
If you're comfortable with ACs, you know the firm well (it's your first choice, one of only a few applications, etc) then you could probably only spend few hours actually preparing. It will depend a little on what they expect you to do at the AC but many things can't be explicitly prepared for like group exercises.
There's definitely a risk in over preparing if it gets you into a cycle of nerves or you're trying to memorise lots of things. One of the best pieces of advice I had was that not all preparation will be reading, researching, practicing, etc. At least some should be just thinking - go for a walk or go somewhere quiet and just take time to think about what you've already prepared, how you might approach tasks or phrase things.