How long does it take for a dog to get an ear infection?

I had my dog's ears cropped (if you want to whine or chastise me about that you can move along to another question now). His ears were wrapped while the sutures were in and I followed the vet's instructions exactly. I kept them dry...I was very careful. I took him in to get sutures out yesterday and have post put in. They said he was good. When we got home I noticed that his right ear was bothering him...I thought that maybe the tape was pinching a bit or the post was bothering him. This morning he started whining whenever he bumped it or layed down on it like he always does for his naps. I inspected and didn't see anything wrong. When he woke up from his nap, a large amount of brown liquid poured out like gravy :( Called the vet an told them. I removed the post and wrapping on that ear like they said to. Took him in...he has an ear infection and has to stay overnight. Should they have seen this yesterday?... Or could this have happened overnight?