SRV, I dont think i'm a shredder (that looks so funny on the screen! heheheheh), but here goes:
Warming up is very important. Just as Jazhombie stated, you have to condition yourself physically and mentally. Just as you have to tune up your up guitar after not using it for a while, you have to tune your physique and mental focus.
I dont' believe in mere "physical" warming up (scales, the 1234 warm up, etc) coz just doing so limits you to only following your fingers. You have to tune your head, follow your thoughts and ideas.
I guess beginners take longer to warm up, while the more "professional" may take a few minutes.
When I mention "mentally" warming up, that means listening to music, imagining and "thinking" guitar even before you pick up the instrument.