At present, the ground study for an IR on top of a PPL is almost as much as obtaining a CPL (or even ATPL) straight away. This is due to the JAA classroom requirements. Expect about a year part-time, or at least several months full-time.
It is recognised that this is overkill if you're not on the commercial track. There is talk of reducing the IR theoretical requirements to a more sensible level and this might happen very soon. Look for a loooong thread on "IMC rating" in the Private Flying forum. That's where the experts seem to abound.
This will not influence the practical side considerably. I think you're required to have something like between 25 and 40 hours in simulated IMC, flying straight and level, turns, climbs, descends and most of all the various procedures and holds before you're allowed to do the IR practical.
BTW I don't hold an IR/IMC rating but I mostly wanted you to point to the other thread. As for me, I'm waiting to see what happens in the next year and a half with this initiative before committing a hell of a lot of money on a (possibly needless) ground study.