The vet told me they should feel better with in a week but you then have to get it done again 3 weeks later when the next cycle starts. I had to get it done again within 6 months and I'm thinking he has them again because a horse came on the yard who I'm positive had mites but was never treated and was in the stable next door to mine and I think he got them again although I might just be being paranoid as people keep telling me. But I've just bought some frontline spray from vets this time.
Just ring your vet and ask Rhea. Surely if they needed to come back in 3 weeks time they would have told you that or booked an appointment there and then ready wouldn't they? I know my vets would have done so maybe you have a different type of jab. Best ask them to clarify x