MP's undergo OSUT which includes BCT and AIT. It is 19 weeks and 1 day. AIT phases will be a "little" more relaxed than BCT. It will all depend on your company. You will have drill sergeants all 19 weeks. It will be more disciplined than most AIT enviroments. Almost like it was in BCT.
If you want a day-by-day account of it, check out my blog archives (next week when I put it back online).
In short, it's three weeks of L&O (Law & Order) which is a lot of classroom stuff, practical exercises and a couple days in patrol cars playing MP. The rest is a mix (roughly in order of time allotted) between detention operations, convoy security, MOUT (i.e., clearing buildings), five-day FTX, unique weekend PT (i.e., "hunger ruck," Demo Hill), crew-served weapons and some other jazz.
As Swortzell said above, it's OSUT so the change from BCT to AIT is minimal. Some extra benefits and perks, but nothing substantial. Some DS ease up quite a bit, others keep it on the whole time. You'll have contact with a lot of "civilian" instructors who are often retired Sergeants Major which takes a little pressure off. And that's about it.