It's very difficult for anyone to say exactly how long it would take. There are many, many factors involved including: motivation, how quickly you learn things, how good your instructor is, how good the weather is where your at, how good the aircraft availability and maintenace is, etc.
It's hard for me to say which one was my hardest. It's a toss up between the Private Pilots License and the Instrument. Private Pilot because everything is brand new to you, and your learning a skill you never before had.
The Instrument on the other hand is equally difficult, because now your learning to control the airplane soley by reference to instruments, and there can be a learning curve. Also, it's A LOT of information to take in and digest, and apply at the same time. With that said, I thought the Instrument rating was probably the most rewarding and funner ratings I ever did.
Multi-Engine training was defintely the funnest, although it was too short (typically takes only 5-10 hours).
Good luck, and be sure and keep us informed on how your training is going.