Custody agreement ambiguity is killing me

Quick question: what counts as "reasonable notice" for schedule changes? Mine says 48 hours but my ex keeps texting me at 9pm asking if she can have the kids an extra day starting tomorrow. I usually say yes because I don't want to be that guy, but I'm starting to resent it. Am I being unreasonable to enforce the actual agreement, or is flexibility more important? This happens like twice a month and it's messing with my ability to plan anything.