I have a '10 Justa 6 speed. I've driven it about 500 miles since I bought it and the oil seems to be about 1/2 qt. down. Approx. how many miles = adding a quart of oil? What's "normal?"

A quart every 1K. I leave mine a half quart short of full and it burns oil a LOT less than it use to. I only filled it a half quart in 3K last time. I check weekly.

My 12 S JCW with 40K gets changed every 5K and is still showing full

Every car and every engine is different. Check it often until you figure it out and feel comfortable waiting longer intervals until checking it again.