quick as a dog can lick a dish. Very quickly.

Dogs are notoriously aggressive eaters who can be relied on to not only eat quickly, but overeat if given the opportunity.

In the children's book Shakespeare's Scribe by Gary Blackwood the phrase is used colorfully, though somewhat anachronistically: “Before it could descend, Mr. Armin was through the curtains and across the boards. As quick as a dog can lick a dish, he had Mr. Hemminger's rapier out of its sheath and pointing to the constable's throat bole.” In the early 17th century “hound” was still the preferred term rather than “dog.”

The phrase turns up in a number of variations.

see also: dish dog; The quick brown fox jumps over the lazy dog.