About 5 minutes at most, it's not pixel art. :V
Lemme illustrate the process... (All done in Photoshop CS4)

Firstly, we take a gray circle.



Then we add some shading with the Burn tool...



Already now it has some depth, but let's add some more shading...



That's enough shadowing, so let's add some light with the Dodge tool...



Almost finished! Now we just add a little reflection, again with the Dodge tool...



And we have a sphere. It's a whole lot easier (and quicker) than making a pixel sphere, really.

By the way, thanks. ;o