When should I use incenses?

I used mine in certain events such as WORLDS Anaheim. It spawned a few extra Kangaskhans for me and I heard a lot of people got Unowns from incense there. Farfetch'd is another example.

Not windy much here, but in winter when it is, cold. I ran some stationary incenses couple days ago hoping for at the least an Abra, nope: pidgey, bat, hoppip and those were the memorable ones times 4. If they are taking up space in your bag try some, but realize you are playing blindfolded darts incensing for a particular weather boost

I use them when I take walks in the neighborhood.

I'm not positive but it sure seems like if you are moving they spawn more Pokemon. My kids have used them while riding in a car (not interstate, around town)

Incense spawns one pokemon either every 5 minutes or one for every 200 meters moved, maximum spawnrate 1 per minute. So it's best used as a passenger in a vehicle that moves slow enough for you to catch everything or riding a bike with a go+.

Everything that normally influences spawns also affects incense so using it inside nests spawns nest pokemon and weather affects incence spawns. It used to give totally random pokemon when used in an area without a natural biome, but weather seems to have ruined that trick.

So use incense a) on a public transport or friend's car on a route where you can also spin stops and catch other pokemon
b) bicycling through a nest park with a go+

I plan on using the remaining ones I possess while simultaneously activating a star piece the next time receive a double stardust event (3x stardust)