What does the Warden mean when she says, "I've waited this long, I can wait another day." What does she tell us?

The Warden is looking for something.... or we might say she has the boys looking for something she wants. None-the-less, the meaning is clear. She wants the holes excavated carefully..... thus, she can wait to make sure whatever it is isn't missed.

"All right, that's enough for today," the Warden said. "I've waited this long, I can wait another day."