Biopsy results?

Does it hurt, and how long does the whole thing actually take? Also—just out of curiosity—how do they even perform the procedure?

If there's an open wound, they're basically scraping off a little bit of tissue
If you're dealing with spots or scabs, they usually run a DIF—direct immunofluorescence. They’ll sit under a special light for about half an hour to warm the area up, and then they go in for the scrape.

Why you might actually need a biopsy for dermatitis...
The way
Susan Scott5 phrased it really nails why you'd go through with a biopsy.

They needed to confirm if it was actually psoriasis, so they sent me in for a biopsy:
They apply some topical anesthetic—though I couldn't use it because of my allergies—then they just take a scalpel and snip out a tiny half-centimeter piece of skin for testing... wrap up my leg in bandages and you're out the door... honestly, the whole thing takes maybe 3 minutes... it just takes a bit longer until it heals up and I can finally get the bandage off...