How long should i use an air cast after a broken foot?

Depends...: An air cast is a form of immobilization. Your use of it will depend on the type of fracture/break that you have and your doctor's decision.

Aircast: Use the aircast til your doctor says you can take it off.

Air Cast: An air cast splint is usually used for a sprained ankle. For a broken foot usually a cast or boot would be used for 4-6 weeks for weight bearing protection while the fracture heals.

To limit swelling.: Air cast are used to stabilize the foot and to minimize the swelling. Healing rates depend on various factors. In general bone may take 4-8 weeks to heal. I am assuming you are referring to a pneumatic walking boot as typical stirrup ankle air cast has limited efficacy for use on a broken foot. The pneumatic boot would be use for the duration of the bone healing time until x-rays show healing.