The answer must depend on where you are now in terms of licenses/ratings.
What you describe will be of little use without an IR, and the ground school for the JAA IR will take you anything up to 2 years if you are say 50 and have a busy life. Or never, if you have a busy enough life
If you are 50, good technical brain, already been flying for a few years on the IMC Rating, and busy, the FAA IR ground school is probably 6 months.
Obviously it varies. If you are a recent Cambridge PhD in Pure Maths then it will be easier
I write this as a nearly 50, busy, pilot of a high perf piston single.