I've aways gotten a health certificate within 15 days of flying with mine. It's actually not a rule but a law. My vet told me that technically you are even supposed to have one if you travel interstate by car. Like the others, I've never been asked to show it but technically it is the law and if they want to they can ask.
From this website:
Traveling with Your Pet FAQs
If you are taking your pet across state or international borders, a health certificate is required. The health certificate must be signed by a veterinarian after your pet has been examined and found to be free of disease. Your pet's vaccinations must be up to date in order for the health certificate to be completed.
Here's another one.
Do I need a Dog health Certificate for my dog traveling by - JustAnswer
Customer Question
Do I need a Dog health Certificate for my dog traveling by car from Wisconsin to Arkansas for a 5 day visit with my uncle ?
Accepted Answer
Thanks for using JustAnswer.com, I'm Dr. Dan and I'll try to help you.
If you wish to follow the letter of the law then... yes. Anytime and every time an animal of any kind crosses any state line they need a certificate of health within 10 days of travel.
I hope this helps, best wishes.
Dr. Dan
For international travel it can be even more strict depending on the country. I've never had a problem with Canada.
The APHIS Vet Health Certificate (Form 7001) for International Pet Transport - Pet Travel Expert | PetRelocation.com