It’s by steps, you have a chance of getting an egg every 256 steps.
That chance is dependant on the parents. Additionally, if you have reached Helios city, you can buy an oval charm in the underground black market to increase the chance.
You’d have to count the steps manually if you want to check exactly every 256 steps which would be tedious. The count starts when there are two compatible pokemon in the daycare iirc.
Alternatively, you can talk to the old man inside the daycare(once every 2 minutes or so in between walking around) who’ll say something along the lines of “There you are, my wife is looking for you” when an egg is available.
p.s.: Breeding guide.