The generally applied "rule" is two seconds of space between cars in
good weather, four seconds in inclement weather or if you're towing a
load (in a trailer, for example). Some websites put those guidelines
at three and six seconds. Here are some sites that should help (note
that they aren't all US, not knowing where you're from, I can't give
you specific guidelines for your location):
SmartMotorist.com: Safe Following Distance
This adheres to the 3seconds/6seconds rule. The nice bit is that they
have a table of distances that translates the rule into space.
New Zealand Transport Safety: Key Driving Skills, following Distance
If you follow the slides you'll get a description of the 2 and 4
second rule as well as translations for the metric system.
City of Toronto Taxi Training: Safe Following Distance
The City of Toronto takes a bit of time explaining why two to three
seconds is the safest distance between cars.
Alberta, Canada Drivers Handbook
Again, two to four seconds of space between cars is safest.
Roadtripamerica.com Rule #13: Create Space: Use the "Two-Second-Plus Rule"
Again, two seconds. Plus be aware of your sides and back. Also
includes a description of why the length rules were hard to remember.
I found these sites by doing a search of the Google database for
"following distance." Please let me know if I can help further!