If you sit the exams a couple of months after starting you ain't gonna stand much chance of passing. I think you might be underestimating the depth of knowledge required to pass the JAR ATPL's.
However, I have never heard of a minimum time requirement. The maximum time allowed is 18 months from sitting your first exam. I know there is a classroom hours requirement for integrated training and a requirement for progress testing but that's about it. Classes get booked up quite a way in advance at alot of schools so I think that'll be your limiting factor.
VFE.