The biggest credit card introductory offers usually don't last long, and could change every 1-3 months. But some credit card deals last for over 12 months, and others are only available through particular websites. So it varies a lot between cards.
If there's an offer you want to apply for, check the date it ends. And if no date is listed, check the details listed on the application page before you apply to make sure it's the same offer.