In A Wrinkle in Time, how do we know that the children have only been gone for less than a day each time they leave? Give textual evidence.

When you ask a question about when the characters “leave” and return in A Wrinkle in Time, you are speaking of the tesseract.  This is simply a way to travel through space and time.  Because the characters in this novel can manipulate time and space, returning in less than a day is entirely possible.  You are correct in assuming the children have been gone for less than a day.  The evidence of the answer...

When you ask a question about when the characters “leave” and return in A Wrinkle in Time, you are speaking of the tesseract.  This is simply a way to travel through space and time.  Because the characters in this novel can manipulate time and space, returning in less than a day is entirely possible.  You are correct in assuming the children have been gone for less than a day.  The evidence of the answer to your question can be found in comments made by the characters that have remained behind.  For example, when Meg finally saves Charles Wallace, the entire group returns home.  They land in the family vegetable garden in the middle of the broccoli.  Suddenly the twins, who were left behind, are running towards them with Dennys yelling, “Hey, Meg, it’s bedtime.”  This is the biggest piece of textual evidence here.  Dennys is not necessarily glad to see them or crying because of their previous absence.  Instead, Dennys is simply doing what any brother would do on a normal day.  Mrs. Murray is running towards the group as well, but for a different reason.  Meg’s father is with them.  Mr. Murray, of course, has been gone for much more than a day.  He is the one the children were trying to save.