YankeeFan and John Bender:
How much time would it take me to become proficient enough on the piano to play some of Billy's songs? I have no keyboard experience but play the guitar and understand some music therory. I can't read music though.
Can you read sheet music? That'd be the first question. If you can or you can learn quickly, you can learn a Joel song at a slower pace but if you just stick to that song, you should be able to perfect it. Practice it over and over and over.
(I would suggest starting with a song in C major, i.e. NY State of Mind... C = no sharps or flats, white keys only).
I've played for 20 years so for me, it still takes a week using sheet music to get it perfect. For simpler songs, I just need what key it's in or a lead sheet and can work it out but would take longer.
Download Sheet Music Direct for your iPad and buy the NY State... Music for a few bucks and give it a whirl.
There's 2 other cheats you could use...
Look for a tutorial on YouTube and just memorize the keys without ever cracking open sheet music (I do this on guitar all the time if I just want to learn a quick song).
Learn to play all the chords in your LEFT hand and most sheet music will give you what chord each bar is in. That allows you to ignore the bass clef/left hand completely and just play the melody in the right hand. Over time you can arpegiate the chords in the left to make it more lively sounding. Rarely do I ever look at the bass clef for most pop/rock music, I play the chords in different ways with left hand and follow the treble clef only.
Anyway, this is boring as all hell and don't want to hijack. If it seems intimidating, just YouTube the lesson. If you don't find one on YouTube, I could make one and upload it for you pretty easily.
GL!