In my experience C-level executives typically stay for 3 years. It will be longer for a founder, but the hired-into-the-role will want to have made an impact and be looking for the next role.
Further the newly hired C level will take between 3 to 6 months before they know the specific business enough to start to make changes.
The outgoing C level may take some of their trusted staff with them. This can cause some disruption itself.
This pace of change could explain the typical 2-3 year duration of developers at a company. The new manger will stop or alter their predecessors projects and start new ones.
Developer Rants 