A reasonable update cycle would be 1-2 updates every year. That means refreshing the hardware every time a new SoC is released or when third-party components (RAM, SSD, display, etc.) become better/cheaper.
I usually keep my Macs 5+ years, but I try to avoid models that are older than 6 months, because old hardware tends to be poor value for money. The moment I update is often beyond my control, which means that a 18-month or 24-month cycle would usually force me to buy obsolete hardware if I want to continue using a Mac.
The only reason the Windows side updates their machines 2-3 times a year is to generate sales. If you walk into Best Buy today and compare the laptops currently on sale to what will be on sale in August when back to school season starts, you will see that the model numbers will be different, but internally there really won't be much difference between the models. There really is no need to refresh hardware that often from a performance perspective. Even if RAM costs were to drop, a manufacturer would need to use up their existing inventory of RAM first before they could start putting less expensive sticks in their machines. Hardware doesn't go obsolete in six months, let alone 18-24 months.