I found
a tunneling article, which references ancient techniques from 6 BC in which engineers through solid rock were documented tunneling around
9 meters per year, (almost 30'). Modern techniques have roughly tripled that rate. It's not a simple matter. You have to remove the earth, account for stability, and make sure fresh air is coming in. This requires a fantastically tremendous amount of labor if you don't have modern machinery, or in D&D some powerful magic.
So, in 3 days, 20 kobolds wouldn't likely make a dent tunneling anything with simply picks and hammers. But, as I think others were hinting, if the narrative fits, then the kobolds have something advanced to get the job done. Maybe they have a deal with an elemental creature of earth, and so on.