So this comes up in sci fi a lot, beginning with "2001: a space Odyssey". In that movie Dave has to jump from the pod to the emergency airlock without a helmet.

He does it and it takes some seconds, approximately 10 or so..

It happens in the Hitchhikers Guide to the Galaxy wherein Arthur and Ford are thrown off the Vogon Constructor Fleet ship and are rescue 29 seconds later by Zaphod in the Heart of Gold.

Total Recall had it where Quaid and Melina were exposed to the partial atmoshpere of Mars for awhile until the atmosphere kicked in.

It's in the new movie Sunshine too.

So how long can a human actually survive in the vacuum of space?

Turns out we actually have an incident, where the astronaut survived.

At NASA's Manned Spacecraft Center (now renamed Johnson Space Center) we had a test subject accidentally exposed to a near vacuum (less than 1 psi) in an incident involving a leaking space suit in a vacuum chamber back in '65. He remained conscious for about 14 seconds, which is about the time it takes for O2 deprived blood to go from the lungs to the brain. The suit probably did not reach a hard vacuum, and we began repressurizing the chamber within 15 seconds. The subject regained consciousness at around 15,000 feet equivalent altitude. The subject later reported that he could feel and hear the air leaking out, and his last conscious memory was of the water on his tongue beginning to boil.

[linky]

Bottom line: you have about 15 seconds until you pass out...and that will result in you...um...dying.

It's a nasty ride, you're blood boils, your skin blisters, you lungs might explode if you try to hold your breath...icky, icky.