Most of our music is pretty advanced, so when we hear something that we really need to work on, we try to start learning it well in advance. Maybe two months. If there's an immediate need for something complicated, we learn it in chunks. An important thing for us, as well as other groups has been to not spend too much time on a piece at one rehearsal. After working on something for a certain amount of time, nobody is retaining anything and you just end up with the same problems the next rehearsal. Work on a section of it, then move on to something else.