I will have to agree with those who choose to bypass a little of the conventional wisdom.
It should be safe to say that extracts are supplied in a ready-to-use form, i.e, Pilsner malt extract. Now if you go cooking it for an additional HOUR, it will no longer be Pilsner Malt Extract. Unless there are no available sugars to be affected by the additional cooking. After all, when using an all-grain recipe, or even a partial, an extract is made from the grain by heating in water. Once the optimal amount of desired ingredient has been converted/released from the grain, a one our boil commences. To boil for an ADDITIONAL hour is not described in ANY recipe I have seen.
My first two extract recipes didn't turn out in any blue ribbon categories. The first batch died of natural causes. The second batch (five years later) had darkened well beyond it's profile and had enough twang to launch an 8 foot fencepost. It took extra time to tame the twang- and was not recommended to impress friends. The batch that left the gate with great fanfare was an American Wheat that was done severely limiting how much extract was in for the 60 minute boil- 1lb. was all I put in. The remainder was added in the last 10 minutes of the boil. The wort tasted appropriate, the beer was ready on it's schedule without additional aging. "Extract twang" almost non-existant.
The most recent batch was done a little differently. I'm still doing 3-gallon boils, and this one called for a full 3 ounces of hops. After two ounces where in, it became obvious that the hops would absorb all of the wort and would scorch, so I boiled a second gallon of water and added the last ounce of hops to that for 5 minutes. This went into the fermenter first; I figured it may be good to have antioxidant properties in the fermenter waiting for the wort. An additional gallon of water form the freezer was added, and brought the temp down to 74F. The wort chiller dropped the temp of the wort in to stockpot down to pitching temp in about 10-15 minutes. The drop from boiling to 100F was the fastest part- it took longer to get the last 26 degrees.
SG points looked right, so I pitched. That was Sunday evening. As of this morning, fermentation has nearly completed, and this stuff smells GOOD! Just took the final gravity and the Cooper's hydrometer reports 1.002. Amazing? No. It is off by .004 -.005 (measured in distilled water), so it is more than likely done at 1.007 or so. Off to aging it goes today. And it has a good beer taste to it. Just way too green!!!
My vote goes for NOT over-cooking extracts.