It’s hard to keep them when they’re starters. Most of the SEO work done is quite different than the one you do for larger organizations.
For starters, you will focus on onsite, fixing SEO issues and building Content on Search Terms. For established ones, you will have to focus on offsite, prospecting for genuine authority backlinks, which looks very much like as sales job rather than background tech.
The only way to stay in a growing organization is to establish a Roadmap from the start, where you’ll explain how the tasks and budgets will grow with the organization.
Starters leave after 6 months in average. Large organization like P&G and such stay for years, they foster the relationship.