Immediately.
According to the 2016 Compendium for Rabies Control, a peak rabies virus antibody titer is reached 28 days after initial vaccination and immediately after booster vaccination.

Here is some additional information on rabies:
When in doubt, it is never wrong to check with your local health department.

Basically all potential rabies exposures boil down to 2 questions:
1. Who is involved?
Animal-animal exposure (less alarm bells)
Animal bites/exposed human (more alarm bells)

2. Was dog/cat/ferret vaccinated?
--Documented up-to-date on rabies vaccination (booster + 45 days owner observation)
--Documented vaccinated, but overdue (booster + 45 days owner observation)

--UN-documented vaccinated, and overdue (Handle on case-by-case basis)
--Ferrets that are OVERDUE for booster (Handle on case-by-case basis)

--UN-vaccinated pet (euthanize or vaccinate + 4-6 month strict isolation)
--Wild animal, esp. bats, raccoon, skunk (euthanize, send head to state lab)

When dealing with rabies questions, ask yourself if this seems like a HIGH-risk exposure (e.g., wild raccoon bites a child)
or a LOWER risk exposure (e.g., up-to-date vaccinated dog messes with woodchuck but no bite wounds on dog).

For high-risk lean towards euthanasia/testing or immediate vaccination + long, strict isolation (4 months, dogs/cats, 6 months, ferrets).
For low -isk lean towards immediate vaccination + shorter, easier observation period (45 days).

In general, both Canadian and U.S. guidance on rabies post exposure management conforms with Compendium guidelines.

References:
United States guidelines
2016 Compendium for Rabies Prevention and Control, JAVMA, Vol 248, No. 5., 505-517, courtesy, Natl. Assoc. State Public Health Veterinarians (NASPHV compendia).
For information on post-exposure prophylaxis in people see: Human Rabies Prevention-US, 2008 ACIP Reccs: May 23, 2008 / 57(RR03);1-26,28.

Canadian guidelines
Canadian Food Inspection Agency (CFIA) rabies home page and rabies testing summary.
The Canadian Veterinary Medical Association (CVMA) on rabies guidance homepage.
Click here for a CVMA post-exposure management summary.
Click here for CVMA post-exposure management presentation.

Global guidelines
WHO OIE World Health Organization Expert Consultation on Rabies (2nd Report, 2013).