How long does it take to cycle a tank using fish food/when to add fish?

Typically the longer the better. You always have to cycle your tank and if you do it longer it wont hurt. I would say wait at LEAST a week. Just make sure you are testing your parameters each day, and once they are fine you could start adding some of your more hardy fish. Always at the most delicate fish last. Also never add more than 3 fish a week because it will cause too much stress.