Hi. I've been trying to learn Cliffs of Dover for years. I started learning it over two (or 3?) years ago and have gotten pretty close, with some mistakes (it's mainly speed).
Here's a link to my cover of it ( I can do better now, this recording was rushed): http://www.mediafire.com/?3xbzvgbojawjmat
My advice is, if you want to learn it, you learn plenty of other songs while you learn it. Learning other songs while learning it concurrently will give you experience in the techniques used by Eric Johnson in the song. Variety is key.
If you haven't been playing for at least a year already, you won't be able to learn it two years. It'll probably take you 3-5 to master it.
The truth is, the amount of years doesn't quite fit in. It really depends on if you play daily. Guys like Eric Johnson, growing up, they played for hours every day, every week, every month, for not years, but decades. EJ was 36 years old when it was released. If he started playing at 9 years old, he had been playing for 27 years; So mastering it in two years might be a little preemptive.
What you want to do is:
1) Have a good guitar (and the right gear). EJ uses a Gibson ES-335 (bigass guitar) on the studio version of Cliffs of Dover. Live performance, he uses a Fender Stratocaster. The kind of guitar you want to have is probably a Fender Stratocaster. Or a nice high end Epiphone (which is owned by Gibson). I DO NOT recommend a Squier or a Starcaster; I've had them before and you really need to do a lot to them to make them worth playing: they're too much of a hassle.
The truth about playing guitar is that the gear for playing will cost you a lot, but the items I'm recommending to you are on the cheaper end, but have very high quality. You will need some money to be a guitarist (you might spend hundreds for everything combined, but compared to thousands...it's a lot better).
For an amp, I would recommend a 20 watt Epiphone amp. Solid state, relatively cheap, but a very nice amp that will last you for years. Do not get:
A solid state 20 watt fender amp: they make a hissing noise and do not work well with distortion/effects.
Any Line 6 amp. Those are made for metal players, and definitely do not sound good with Eric Johnson style playing.
Any giant tube amp. They're expensive, loud (and that will piss off your parents), and hard to move, especially for your size. Do not get a tube amp.
For distortion/effects, I recommend getting an RP355, because it will give you the perfect taste of all the amazing effects you can use as a guitarist. Plus that will teach you what sounds good, how to dial in your tone.
And above all (except for having a good guitar of course), you must have a metronome. You must be able to keep yourself in time for playing a song.
2) Learn the basics. Everything. You must learn guitar before you learn Cliffs of Dover.
Learn about: Parts of the guitar (and their functions), tuning, notes, chords, scales, a little history of rock and roll, some famous rock and roll songs.
3) Learn how to play famous songs (and DO NOT jump in over your head. You've gotta learn Smoke on the Water long before you learn Cowboys from Hell.) If you ever tell someone you play guitar and they have one for you to play, you GOTTA be able to perform something they know, otherwise you'll look like an idiot.
4) Do not join a band until you've been playing for at least 3 years. Bands are filled with false hope of someday stardom, drama (if you start dating the drummers' ex-girlfriend and that splits the band apart, other crap). A big problem is practicing with the band, because practice times might not always fit into your schedule, maybe your band members are either too serious or don't even care anymore about playing. But the biggest problem is when you realize that either you, or the band is starting to diverge from the musical style that brought you together: "Hey let's make a band and cover AC/DC!" "Yeah, that sounds great!" (One year later...) singer: "You guys, I think I have to leave the band because I'm more into screamo than ac/dc." drummer: "I was about to say something similar dudes, because I've gotten into the punk stuff like the ramones now and I want to play with members who play that kinda stuff." and the bassist and if you have a keyboard guy, they end up saying something like that too, and now all the time and effort that you put into that band is wasted, because you missed out on putting your time into something else and that band is going nowhere. Bottom line: Wait awhile before you join a band.
Remember, learning guitar is a quest: You will battle dragons before you can save the maiden.