How fast did your baby/toddler's feet grow?

DS is 11 months and in a size 4 shoe. I'm just wondering how quickly he'll outgrow sizes. Will they last a whole season? How do you know when to size up? I want to buy DS a pair of sandals for the summer, but I don't know what size I should buy or if I should just wait until it's warmer and he'd be closer to wearing them to see what size to get.