Read my recent post, I sort of dive into this subject a little:

http://www.guitartricks.com/forum/showthread.php?t=32299

Here is another link to Van Halens wiki page. It says he would literally lock himself in his room and play for hours.

http://en.wikipedia.org/wiki/Eddie_Van_Halen

When you practice, your gains are exponentially. Meaning if you keep on it, and practice a few hours a day, you can learn all those songs in a lot less time than 2 years. Although probably not have the solos perfected. Or maybe you can if you practice enough!

So I would say yes, it pretty much is grinding down the hours. For the famous musicians that do pull out the 8-10 hours a day, I bet its something they love doing. I bet some cant help but to play and practice for that long. They need to play guitar just as much as they need to sleep.

I think on John Mayers wiki page, I read that his parents actually took him to a psychiatrist on two occasions because they thought something was wrong with him for playing the guitar so much.

Also keep in mind, what you practice specifically makes a big difference in how quickly you will progress. I would say thats what this website is for, to help you build a solid foundation with no cracks in it. My overall guitar foundation definitely has cracks in it still, and I have been playing for 8-9 years...