BOLC is an every changing beast. Even the name changes every few years it seems (OBC, OBLC, BOLC, etc.). I can only speak on my experience and from reports I've heard from others, but the best info comes from the HPSP program itself (I think USArmyHPSP is actually a poster from the program). When I did the program it was hours of online training and then 26 days of in-residence training at Fort Sam Houston. However, normally that short course is for people that are in the reserves and the rumor at that time was that they were going to disc. the short course and have all HPSP students do only the long course. We were a beta test basically.
As for when to do it I'd highly recommend waiting til after school if you can. You learn some good Army tools that will quickly disappear if you go back into school. Your school also matters because my school didn't have a summer break so we didn't have a chance to go while in school at all.
End of the day BOLC was a ton of fun and having done the 3 week course I wish it would have been longer.
Good luck!!