It started out as every 12 months in the original 1965 article which lasted for about a decade before being updated to 18 months, and relatively recently people started talking 24 months.
In the early 1980s there was a time it seemed to hang in the air because all the clock rates in a micro were synchronized to the video scan. Steve Jobs had no idea the Apple ][ was going to last as long as it did so they came out with the the failed /// and the 128k Max that was really just an awesome demo.
It wasn’t until the mid 1980s that PC compatibles reached a place where you could buy a new computer that was exponentially better than the computer you bought 2 years ago but that was gone around 2005. Until then the smaller transistors were really faster and consumed less power but since then you mostly have more of them and moderate improvements in power consumption.