Not much. Last summer DS started off the summer in his sandals from the previous summer. They were a size 12.5-13 if I remember. He grew into a 13 - 13.5 sometime in the fall and still fits in them fine. I figure by next fall he'll hit a size 1. DD has worn the same pair of sandals for 2 summers running now, and they still fit though she may outgrow them this year (hopefully! She has a pair a larger size, but never needed them!). At closing in on 6, she's got tiny feet and wears a size 9-9.5. I see her firmly in a 9.5 by fall.
ETA: But on the flip side, one year my cousin was complaining that his tennis shoes were too tight. My aunt was annoyed as she'd bought them at the start of summer and it was only August, but she went to get him new ones. When she had his feet measured they discovered he'd grown FOUR sizes in 3 months!