Instrument ratings
Hi Tomasz,
Your instrument rating for a rotorcraft will depend on what issuing authority you do it under.
Under FAA rules, where you can fly in a single engined piston helicopter, it will take a disciplined student just under 2 months (however I have known people take up to 6 months) and cost roughly $8K to include your materials and flying if you can complete it close to the minimum requirements (not too difficult)
Under JAA rules, the choices are much more limited, you MUST flying in an IFR turbine helicopter (MUCH more expensive), and the ground portion of the rating is more arduous. The cost of this is a lot higher, starting at around $45K for an ab initio student. This cost is very much dependent on experience and ability, for example if you have a recognised ICAO instrument rating and considerable actual IFR flight time it may cost much less.
The instrument rating is proof that you can satisfactorily navigate and manouver the aircraft by sole reference to the flight instruments, cope with emergency situations, and demonstrate adequate knowledge of the airspace system and navigation aids available to you, varying depending where you will be flying.
Many jobs require one, although often only for insurance requirements.
Some find it very difficult, others find that it is fairly easy, depends on your abilities and mentality. If you like systems, simple mathematics, and have a good memory it should pose no major problems for you.
Good luck with your studies