Maybe every 5 or so years, and generally there has been something worth upgrading the mobo for like new connections for storage. So far it has been when it struggled with 75+ FPS in games that I care about at the settings I want.

Since it is so spread out I can't say it is a solid pattern, but so far each CPU and mobo upgrade have been together with a new set of RAM and occasionally I get extra RAM in between. Hard drives/SSDs and GPUs are whenever but generally they are years apart too.