Specifications for many GPS receivers indicate their accuracy will be within about 10 to 50 feet (3 to 15 meters), 95% of the time. This assumes the receiver has a clear view of the sky and has finished acquiring satellites.
Many receivers include WAAS capability, which can enhance accuracy in many parts of North America. But if you're moving around or in areas with less than ideal conditions, you'll probably find your receiver isn't using WAAS a good share of the time.
All things considered, you can usually expect to be within about 20 to 30 feet of the mark with most consumer grade receivers.