Sinfully often. Especially on the internet. As far as I see it, if you don't bother to proofread, you're either too lazy or too busy. Once I get to know someone, though, it's easy enough to overlook.
I have quite a few favorites; they tend to be spaced out pretty evenly (about one or two per year). Chronologically (from earliest to latest), I believe my personal favorites are: Zyzzx, Spin Me Right Round (TwistBit Remix), The Eighth Night, The Memetastic Mr. Sheep, my FE: Fates remix, and Ghost Sandwich.
I usually start with drums, although if I get a really good bassline idea, I throw that together before laying down a beat. The drums are usually really important, because they drive the track and flavor it throughout. Songs have taken me as little as 48 hours, although I don't usually rush so much these days. I've even let ideas marinate for two months or more before finally laying them down. It's often best to have at least half of a song planned out in your head before you actually get to work composing it; otherwise you're left floundering with a half-baked idea, and you're back at square one.
At least that's my experience.