I bought this mattress topper for my son’s mattress. His mattress was a little bit too firm so we decided it needed to be softened up. When the box arrived Kai had so much fun watching it expand. It did not take long at all. But we waited about 24 hours to be sure that it was fully sized out.  I think he had the best night sleep ever. He slept through the whole night and did not wake up cranky. That is the best part. ❤️