Can anyone give me a guess as to how long it will take to take the metro from George Mason University to Union Station on a Saturday afternoon in August?

about 25min

http://www.wmata.com/rider_tools/tri...ripplanner.cfm

Looking at the metro site it looks like u need to take buses to get from GMU to Union Station. Here's the Metro/Bus site
http://www.wmata.com/index.cfm

oops, my mistake -- the trip planner says for George Mason Arlington to Union Statiion there is metro.

Which campus. If from GMU in Fairfax, it will take a good 45 minutes. From the Arlington campus, 25

Fairfax, thanks for pointing that out, I didn't realize there were 2 campuses.

As already noted you'll need to get a bus from the Fairfax campus to the Vienna metro station. The GMU website has good info regarding which buses to catch. The Fairfax County bus system might be a better option than WMATA.

http://www.fairfaxva.gov/CUEBus/CUEBus.asp

Also, if your destination is time critical I'd give it a little more than 45 minutes door to door.

There's been lots of track repair work on weekends, so check Metro's website to see if it affects the lines you'd be using (Orange and Red). That might add another 30 minutes.