Blimey, shouting in your first post!?
Some people will go for maybe an hour, others for the whole afternoon. Generally, the afternoons are busiest and the weekends will be the most crowded; mornings are usually quieter.
As for food, well, if you’re able to do hot snacks as well as cold ones, then I reckon typical street food would be popular. However, crisps, cans of fizzy drink, and chocolate bars would be ideal too. All of this becomes FOD (Foreign Object Debris) near an airport, though, so you’ll need to ensure your customers behave and take their rubbish home otherwise, the airport authorities will see you off.