I'm in a test org in MS and we try to get new hires to check in test code sometime their first week. As others have said here, it's a multi-purpose task: Get the new hire up to speed on how to work with our internal tools, work out any kinks that they would run into anyway, and give them a feeling of having accomplished something for the team.

The actual quality of their check-in shouldn't be terribly important, as what they're checking in is usually something very simple.

Doing it all in a single day is aggressive, but doable. I suppose you'd find holes in the new-hire documentation pretty quickly.