With how good Apple is at re-using their technology, quite some time. You see the camera modules re-used in multiple devices, the processors handed down from iPhone to iPad to Apple TV to HomePod. etc.

Plus, they are clearly still selling a lot of units each year. With less physical changes, they don't have to re-tool all of their manufacturing each year.

With less changes comes more efficiency and reduced costs, which means while less units may sell, they probably make more per unit sold.