Check out this guide for Apple product life-cycle advice

MacRumours buyers guide is a site you should always visit and check things out before you buy an Apple product. This site gives you info about all the various Apple products such as when it was released, how long since it was updated.

Further more beyond just info about the release dates, the website goes on to link you to the various info that is related to the product from their main MacRumours site. Which is a site you should visit regularly to check on apple related info as well. I will introduce various useful websites for you along the way as well so do check here out regularly.