DS just went through it. I Day is long and hard. You are getting yelled at and you have no, like ZERO, autonomy. That is hard on humans. So I day will be stressful because of all of that. I assume all Academies are roughly the same in this regard.
Day to day can vary. Somedays you have evolutions (shooting range, o-course, e-course, etc.). Other days are lots of company time - which DS said was probably the worst. Folding socks, screwing up, getting dropped, etc. "Gray time" it is called apparently. Side note: DS said he could tell at lunch if the afternoon was going to suck or not because the Detailers were in a better mood if there was an evolution. If it was company time, they were more quiet (except for one - he said he could read him at all).
As others have noted, it is pretty difficult to prepare for. Just make sure you are in the best shape you can be without exhausting yourself before your show up (don't run 3 marathons a week in preparation). Do lots of bodyweight exercises and running. Then just roll in and try to keep your sense of humor about it all. DS was pretty surprised how fast it all evaporated once the Brigade reformed. He suddenly had free time where he could walk around the yard and not have to keep his eyes forward. He has a lot more autonomy now, but he is very busy so has to direct himself.