Short Answer: About 300 DPI
Long Answer: A document printed by an inkjet printer is printed with about 300-700 DPI of information so scanning at much higher resolutions doesn't yield a better quality image. However you often won't be able to notice the difference in the resulting scan when you scan at higher resolutions so 300 DPI is about right. Some scanners scan in increments of 200 dpi or 400 dpi so scanning at those resolutions will be about the same.
VueScan is the world's most widely used scanner software,
working with over 5000 scannners Windows, Mac OS X, and Linux.