Mrs. Whatsit says,
we'll have you back about five minutes before you left, so there'll be time to spare and nobody'll ever need to know you were gone at all.
Mrs. Whatsit tells them this after they have all tessered, or used wrinkles in time, to travel across vast distances of space. They are about to visit with the Happy Medium. The Happy Medium, dressed in purple robes and a mauve turban, will use her crystal ball so that they can see home despite being so far away. Here, in the crystal ball, they will see the Dark Thing (evil) that partially covers the planet Earth. They need to learn about this force, which has been battled by great religious sages, great artists, and humane people over the course of Earth's history, so that they can fight it in Camazotz.
Of course, to us as readers and to the children, they seem to be gone for a long time! The fact that Mrs. Whatsit can get them back to Earth five minutes before they left is truly fantastic.