I do gallon batches, I think for the most part my primary fermentation is usually done in around 4-5 days. But I still leave the batch for a 2 week period so the yeast can clean up diacetyl. Also just because it works easier with my brewing schedule to do 2 week primaries. You want to be careful overpitching though, as you may lose some of the flavors that show up in the initial stage when the yeast are multiplying. All depends on the style really. I can't say if the beer ferments faster, as I don't mess with gravity measurements until the inititial pitch, and then close to bottling time.
If you really want to know though, just take daily gravity measurements.