Never! It's never enough! Well, okay, sometimes it is. If it's burning you out or you are damaging your tendons, it's too much. (See 10. When is pushing the limit okay, and when is it not okay? ) I'd simply advise you to practice as much as you can and want to. Basically, the more you practice, the better you're gonna get. Show me a guitarist who has played 20, 000 hours and I'll show you one damn fine player!
In years past, I went through periods where I'd practice all day, then go to band practice and play all night. But you don't have to practice 12 hours a day to get good. On the other hand, I've also gone for months without even so much as touching a guitar. Needless to say, that ain't the best thing to do for one's technique. But the fact is, it's a lot like riding a bike. Yeah, the edge comes off, but you never really forget how to do it. With the right approaches and several days of focused practice, I'd get right back up on top of it again.
In general, it is certainly best to be consistent. Not a requirement, mind you, but an ideal. Strive for at least a little practice every day. For beginners, maybe just start with about an hour a day. Then, increase it from there if and when you can. On the other hand, maybe you can only practice for 15 or 30 minutes a day. The point is, however, that if you do it regularly you are going to improve.
Some of you will tell me that you just CAN'T arrange your time to practice more. This is just so much BS! Unless maybe, you are in prison. What you are really saying is that you have other priorities that you value more. That's okay. Maybe that's good. Maybe you have kids and a family to support, and they come first. Just don't gripe about it, because it is you who ultimately has the power to set your own priorities. Choose wisely, and set them the way you think is best for yourself and those around you. Then shut up and be happy! Problem solved...
Try to be consistent. Try to practice every day. Practice as much as you can and want to. Try to start with at least an hour a day and take it up from there. Basically, the more you practice, the better you're going to get.