Can beer ferment too fast?

With previous recipes I have brewed, I experience 7 to 10 days (give or take) of fermentation.

However, my most recent, and first attempt, porter brew seems to have completed fermentation in 48 hours. My recipe is from the Homebrewer's Recipe Guide Viennese Spiced Porter and ingredients below. What say you? Is there a negative to fast fermentation when a recipe says keep in primary for 7 to 10 days?

http://thedailyhomebrew.com/2010/10/04/viennese-spiced-porter/

The Ingredients

* 6 2/3 lbs DME
* 1/2 lbs chocolate malt
* 1/4 lbs black patent malt
* 1 lbs rolled oats
* 2 ounces Mt. Hood hops
* 1/2 ounces Fuggles hops
* 1 packages American Ale Yeast
* 1 vanilla bean
* 2 tsps almond extract
* 3/4 cup corn sugar

the hotter you ferment it, the faster it ferments. i never leave anything in the primary that short, either, unless it's a hefeweizen, and then it's 11 days. i'll leave a porter in the primary for 3 weeks. yeast needs to eat the sugar, then it will eat chemicals that cause off-flavors for the next few days. i have a porter i brewed in july still aging and untapped, but i took it off the yeast in early august

I was wondering the same, actually...I just brewed a wheat beer

(http://hopville.com/recipe/352136/home-brew/winter-wheat)

and same story...from 1.051 > 1.011 in 48 hours...and that's at 23C! I've fermented hotter and had lag times as long as that before. No idea what's happening!

I made a wheat last year and it was 10 days from kettle to keg.

There's nothing wrong with a fast ferment. Some yeasts will reach full attenuation in 24-48 hours. Some won't. It's not the time that makes for problems, it's other factors, like heat, etc.

Bob