As a user said below, you wouldn't need cross platform support, incremental builds, tests to run, etc.

Just getting the code to compile, link, baking in assets, etc. For a single architecture is a much more reasonable goal.

I'd imagine it'd be on the scale of 1-3months for an engineer to get working full time, but large error bars around this figure

You’re also not trying to get a full CICD pipeline complete with unit/integration tests, crypto signatures, ability to flip features on and off with a click , monitoring of the cicd pipeline, scaled so 1000s of engineers can work at once etc

Microsoft uses Azure DevOps for products as large as Visual Studio.