Yep like Dr. Steve says.. it is pretty much up to the vet...the USDA does suggest a period of 30 days for the titer... but it is not an absolute.
though make sure the pet is microchipped prior to the vaccine.....
Here is the info from the USDA:
(I do note that they still use the Pets Certificate and not the EU Passport. I have no idea what to advise you on this aspect as I used the Cert to come in in June.)
PETS SCHEME FOR DOGS AND CATS EXPORTED TO THE UNITED KINGDOM
1. An accredited veterinarian must certify the identification, rabies
inoculation and results of rabies testing. The PETS certificate bearing the VS
logo, available to accredited veterinarians through the Federal Veterinary
Services offices is the only certificate which can be used.
2. Microchip identification must come first. Only microchip identified
animals will qualify for entry. The accredited veterinarian must certify
he/she verified the chip number. (UK port officials will have available
readers which can read microchips conforming to ISO Standard 11784 or Annex A
to ISO Standard 11785. If other chips are used, owner will be obliged to
present the proper working reader to the port inspector on arrival into the
UK.
3. The microchip-identified animal must be properly inoculated against rabies
by a licensed veterinarian using an approved labeled vaccine.
4 After rabies inoculation (an interval of 30 days is suggested), the
microchip-identified animal is subjected to a blood sample to confirm that
vaccination has resulted in a satisfactory titer. The Veterinary Diagnostic
Laboratory, Kansas State University is the Laboratory of choice for
nonmilitary US and Canadian pet owners. The certifying veterinarian will
confirm microchip identification at the time the blood sample is collected.
5. Once the laboratory verifies that the microchip identified-animal has a
satisfactory level of antibody against rabies of at least 0.5 International
Units per milliliter, the PETS Certificate may be issued by the accredited
veterinarian. The microchip identification will be verified at the time the
certificate is issued. (The certificate will indicate period of validity
starting 6 months after the date the rabies blood test was drawn and ending on
the expiration date of the rabies vaccination. Certificates can then be
reissued so long as the rabies vaccinations are updated prior to expiration,
and the replacement certificate will again be valid as long as vaccination is
current).
6. In addition to the PETS Certificate, between 24 and 48 hours prior to
being checked in for departure, the pet must be presented to a licensed,
practicing veterinarian who will read and verify the microchip number, treat
the animal for ticks and tapeworm using approved products, and issue the
Certificate of Tick and Tapeworm Treatment. Both the PETS certificate and the
Treatment Certificate must accompany the pet to the UK.