My daughter is 2 and a half and has to take miralax (polyethylene glycol) every day. How many days is safe to go without a bowel movement?

Constipation. Constipation is fairly common in young children. It is usually due to low fiber diet or decreased liquid intake. Miralax (polyethylene glycol) works by drawing water into intestine to bulk up stool. A child on MiraLAX (polyethylene glycol) should have soft stool every 1-2 days. If child remains constipated, please see your pediatrician fir full evaluation.
Based on symptoms. The short answer is as long as it takes before symptoms develop. There is no set-in-stone schedule for bowel habits. Once pain, bloating, discomfort develop from constipation then it's probably time to take action.