Apple products are expensive. They re also very difficult if not impossible to upgrade. So before you spend all that money on your shiny new iThing, don’t you think it would be a good idea to know when Apple is going to being out the next one, so you’re not left behind?
Macrumours has a handy buyers guide which details how long it has been since the product you want was last updated, and what the average time between updates is. It also lists all the recent rumours they have around the product. A very useful site, always check it before you buy.