It's hard to say exactly. Some episodes were completely new and some were near-perfect adaptions of the comics (like the Jack Kirby episode), but a lot of episodes are extremely debatable. The City at War arc, for example, was only loosely based on the City at War arc from the comics (and parts of that storyline were already used for different things earlier in the series). Other times, you'll get an episode that has a lot of obvious comic homages that doesn't match up to any specific comic story. In a few cases, they even used a comic story as a B-plot in an otherwise original episode.
Generally speaking, the longer the series went on, the less it started taking from the comics.