I did it in 15 hours (rather to my surprise) I would have thought 20 to 25 was reasonable and it was I expected. I used the excellent ground school facilities at White Waltham as well as Thom and the IMC Confuser. I've also used Microsoft flight sim to practice procedures.
I use the IMCR in anger as much as I can to keep current.
It is essential to thoroughly understand all the theory and procedures not just pass the exams and flight test if you intend to use the rating in anger, but whatever you do undertaking the training will make you a better pilot.