We don't know
His plan is to snap his fingers, kill half of all life in the universe and then sit back and bask in the glow of the gratitude of the remaining population.
This is ALL LIFE including plants, animals etc. as was recently confirmed by the creators.
In an interview with Birth Movies Death, Marvel Studios CEO Kevin Feige confirmed that it wasn't just people that died with that finger snapping. When asked about plants and animals, Feige confirmed that half of the plants and animals of the world are now gone, too. He said, "Yes! Yes. All life."
There is no indication of specificity in the movie or targeting planet by planet in the snap. What we see is it's effects on the heroes we know, probably for scale reasons.
Given the sheer amount of life (not just people) in the universe going planet by planet in the snap would probably not be ideal but then we don't know how the Infinity Stones actually work.
Certainly, before he obtained the Stones he was proceeding on a planet by planet basis because that's the only method available to him which is, I feel, the source of the confusion.