You don't need an incense to do it. Random pokemons can pop up from those spots even without it, incense just increases the chance of it.
Anyway, I ran around for 6 hours today chasing pokemons with my screen on the entire time. Portable powerbanks ftw.
Edit: I've a question; is there a time limit for some of the rarer pokemons? Pidgeys etc never seem to disappear from the list, but earlier today I had a a ditto show up on my nearby list. I tried to find it, but eventually it just disappeared from my list once I started getting close (was 1 paw away). And from my understanding Ditto is really fucking rare in this game, so I was disappointed I didn't even see it spawn.
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