Cooling your wort on extract brews.

What would happen if the wort on a prehopped extract brew were not cooled fast enough? How fast is fast enough for that matter? Will there be in discernible taste differences in my final product?

the reason you want to cool the wort as fast as possible is to limit the time it is exposed to the air to avoid any possible stray or unintended yeast strands form getting in, which can cause many undesired tastes in your brew. The quicker you get it cooled, pitched and sealed the less chance you have for what we call an "infection"

Well - that is "A" reason but not the main reason you cool wort fast.

As Palmer says

The real reason is that sulfur compounds will continue to evolve from the wort while it is hot. If the wort is cooled slowly, dimethyl sulfide will continue to be produced in the wort without being boiled off thus causing off-flavors in the finished beer. The objective is to rapidly cool the wort to below 80°F before oxidation or contamination can occur.

Rapid cooling also forms the Cold Break. This is composed of another group of proteins that need to be thermally shocked into precipitating out of the wort. Slow cooling will not affect them. Cold break, or rather the lack of it, is the cause of Chill Haze.

I have heard these reasons and I too tried cooling my wort as quickly as possible, but I am also wondering about some sort of time table. How fast is fast? 5, 10, 30 minutes?

under an hour is rule of thumb, but quicker is better (unless you're using the no chill, australian, technique).
With a home made immersion chiller I get 5gallon full boils to pitch temps in 20min...

5 minutes is FAST! You should be looking at anywhere from 20-30 minutes to get your batch cooled down. A simple ice bath while stirring the wort occasionally should do the trick. Or... you can just by an IC from your LHBS for about $50-70 that will bring your wort down to pitching temps quite quickly as well.

When I was doing partial boils I could cool 1/2 batch to 90 in 25 minutes and then dilute with cold water.

With a wort chiller it's still about 25 minutes with a full batch depending on the ambiant outdoor water temp.

I would start to get concerned a little at the 45 minute level because of contamination but . . . it really is hard to mess up beer if the recipe is good.

What will mess up a beer is combining many little things not done right.

Making beer is like changing a tire. You can leave one lug nut off without any real damage but leave then all all and you got trouble