The drench is given for up to 3 days, the medicated water needs to be given for a full 5-7 days.
Coccidia are everywhere, some environments are more conducive than others, but it's a worldwide thing that even costs the commercial industry a lot every year. Any one who has chickens will most likely deal with it at some point. Chicks are most vulnerable.
The particular strain of coccidia, the individual immune systems of the birds, and how much damage gets done to the gut before treatment starts are all variable, so time to recover can vary as well. I often see improvement within 24 -48 hours, but some may take longer. It's important to complete the complete treatment time, even if they look better, to make sure it's gotten all of them.