I've had students come to me that spent hours a day practicing a specific thing, and never got anywhere. After I showed them a few strategies for how to improve the quality of their practice time, results started showing up immediately.

So, I'll change my previous statement, because it's missing something... I agree with both Sebastian and John of course.

The more you practice, the faster you will become great, IF AND ONLY IF you know how to practice effectively. If you spend all your time practicing something incorrectly, you'll never really get anywhere, no matter HOW long you practice...

Sadly, this is why most of the common myths and misconceptions about guitar playing and how to become a great guitar player have been spread... :/