A senior engineer on your team about half the week, every week, without a hire.

This suits a product that is live and needs someone who knows it - shipping features, fixing what breaks, keeping the infrastructure and dependencies from rotting, and being the person who answers the technical question when it comes up. It also covers the work nobody budgets for and everybody needs: upgrades, monitoring, the migration that has been postponed twice.

Roughly twenty hours a week. I keep context between weeks, which is the whole point - you are not re-explaining your system every time.