I've found that, for me, an hour of focused practice is far more productive than four hours spent noodling in front of the TV. If your time is limited, make a timetable, and make sure you get through it everyday. It feels really nerdy, but you'll get results far faster this way.
It's also important to be imaginative with your practicing, meaning don't just play the same stuff every day. Work at the things that are hard. For instance, if your timetable says 'ten minutes of alternate picking', don't just play the licks you know you can do, look for any patterns that give you problems and practice those 'til they're as natural as your 'stock' licks.
There's a lot of good advice on practicing in Troy Stetina's 'Speed Mechanics for Lead Guitar' book.
Originally Posted by RAP
I'm 15. I have a pretty active social life though.. What do you guys do if you're out all day? How many hours do you get in then?
I hate to say it, but it's a personal thing about how you choose to prioritise your life. During my teenage years I barely ever went out due to my practice regime, and I don't regret it for a second. You're unlikely to have the same amount of free time again once you reach adulthood, so if you want to be an accomplished player you should really make the most of that. Socialising can come later.