I've heard that a helicopter costs about 900$ an hour for the police department. Unless there are many other people growing MJ in your area, they will probably not fly close to you.
However, ALWAYS remember to bring back all containers, plastic bags, candy wrappers etc. Those things are way more easy to spot than some weed.
Also, you want to avoid creating paths to your plants. If you visit more than once a week, always take different routes to the plant. Paths are easily spotted from the air.