I looked at doing the JAA IR a few years ago.
My estimate, having seen the ground school, sitting on the shelf as approximately 2ft wide stack of A4 paper, was around 2 years for somebody with a business, a life, an ex wife, and stuff like that. I have a BSc in electronics, have designed hundreds of electronic products, and could certainly design a GNS530, hardware, firmware, the lot.
That's the problem: nearly everybody who has the income to play the private GA game at that level (no pun intended) is a successful professional or business person, not exactly young anymore, and they can't just carve out a chunk that size out of their life.
I personally know a few exceptions, some prominent on these forums, but they are either exceptionally academically proficient individuals, or they did the European IR many years ago when there were easier routes, or both.
Nowadays, of course, most serious private pilots go the FAA route, but if that was not available, the IFR GA scene in Europe would by now resemble the drivers of the rechargeable electric trikes on Bognor Regis seafront.
Most people doing the JAA stuff are would-be airline pilots, and most of them have plenty of time, and little money, which is just as well otherwise there would be a dire shortage of 737 RH seat occupants willing to work for £25k p.a. having spent 2x to 3x that getting there.