It's not just a matter of how much time you practice to become a "professional"- natural ability is the largest factor in the equation.
Compare it to sports. You could practice hitting golf balls 12 hours a day and never make it on the PGA Tour.
There are some great points here. Practicing isn't just about time - it's about concentrated time.
I know plenty of guys who say they practice 8 hours a day. In reality they practice about 2-3 hours a day and fill in the rest of the time with breakfast, lunch breaks, cigarette breaks, phone calls etc.
There are several practice related things you can do that might help right away.
1. Have clear goals of what it is that you want to do. Know what you're going to practice, and why. (i.e. I'm practicing scales - but I'm planning on using them in a solo.) If you're practicing something just because you think you should do it rather than having a reason - it will mike it a lot harder to maintain.
2. Try to practice the same time every day. It will help reinforce the habit. Additionally try to find a quiet well let place where you're not as easily distracted.
3. Write it down. When you practice something - document exactly what you practice and for how long. (Ex. Bm Arpeggio 7th position - 140bpm 10 minutes). As you write it down you can actually see what progress you are making.
4. Practice with a timer. Set a timer for 5-10 minute increments and practice specific things. Don't joke around. When the timer is on - focus only on what you are doing for those 5-10 minutes. Pay strict attention to the 3 T's (Timing, tone and Hand tension) - see the posts below. This means use a metronome or a time keeping device as well.
5. Learn songs and solos - in addition to practicing scales, chords, what have you - learning things by ear make you a more flexible musician. Additionally it's important to see how chords and solos work in the context of songs.
Here are some practice related posts that may help as well:
Proper Posture Is Required For Proper Performance – Practicing Part...
Tension And The Soda Can Or Practicing Part III « Guitarchitecture.org
Definitions and Documents Or Practicing Part IV « Guitarchitecture.org
Practice what you play or Practicing Part V « Guitarchitecture.org
Testing Your Vocabulary Or Practicing Part VI « Guitarchitecture.org
Possession Is 9/10s Of The Law But Perception Is Everything Or Prac...
Warming Up: Finger Exercises, The 3 T’s And The Necessity Of Mistak...
This is a HUGE topic - and worth addressing the right way. The hand tension issue alone is one that doesn't get covered enough.
I hope this helps!! If you have any questions about any of this - just let me know.
Thanks Terry - It's just a matter of coming up with a game plan and sticking with it.
Self discipline does not come naturally to me - so the only way for me to kick myself into gear is to set up situations (gigs, rehearsals, recording sessions, songwriting sessions, etc) that force me to hone certain skills to get through the situation. But in terms of sitting around and thinking, "I should really work on my melodic minor modes" I can't really do that. But if I recontextualize it into, "What on earth am I going to play over these solo changes in this tune - maybe melodic minor..." then it happens.
Good luck and stick with it! Hopefully the tips help!
Daniel Levitan in his book "Your Brain on Music" states that mastering anything whether it be the guitar, chess or even cooking, requires 10,000 hours of intense, engaged practice. So if you practice 3 hours/day 365 days per year that would be 1,095 hours/year. So roughly and assuming you miss somedays that's 10 years or more of practice (but again the emphasis is on "intense & engaged"). I believe this to be true, especially in my own case where I left off playing guitar regularly until about 10 years ago when I started to play almost every day and my playing has seriously improved as a result. However many insightful things have already been said in other replies to your question about WHAT you should do during a practice session. In general I work on my weaknesses more than my strengths, trying to see if I can do something I wasn't able to do yesterday. I start slowly and gradually challenge myself more each day until I'm satisfied with the result. Then I move on! Hope this is the kind of thing you are looking for.