The work some of these guys put in is unreal. In the new KC doc, the interviewer asks him about practicing, and he said he practices about five hours every day. The interviewer said something like, "Do you find it ironic that the guy who has the most secure position in the band practices like he doesn't?" But Fripp said later he's very demanding - he said it makes him furious when the band members don't treat the music with the respect he thinks it deserves. It's one of the things that makes the Sunday Lunch videos so enjoyable, because it allows his love of music to shine through - just the sheer joy of playing. Mistakes and all.
John 5 is another guy. Constantly playing. He's said in the past that he works out his songs until he can go into the studio, play one take, and nail it. He said he'll practice his full set in the dark until he gets it perfect so he can play w/o looking at the neck.
My friend has a cousin who played French horn professionally. She practiced about eight hours a day because she had to keep up with everybody else.
It's a level of dedication not many of can afford to commit to. Or even have the ability/mindset to commit to.