How often does incense spawn Pokemon?

If you're standing still you can get one every 5 minutes. If you travel at least 200 Meter per minute you can get a Pokemon every minute.

Rate of movement does not affect incense spawn rate. It just means you're picking up ambient pokemons in addition to those lured to you. I use incense while sitting in my bedroom, which hardly ever has a spawn. It's close to tared for zero.

In my experience, incense lures about 4-5 pokemons per pot. Most commons. Sometimes an uncommon. Once a rare. Probably RNG as to when and what, but I've never had less than 4 or more than 6 come for an incense.