It depends on the animal's vaccination history. If your animal is current on rabies, you can just do the microchip, re-vaccinate, and do the blood draw in the same day.
I ended up waiting about 12 days after the rabies (though my guys have a current 3yr vaccine from Nov, they got redone in May b/c my cat's microchip disappeared), and they were about 5x the minimum required level. My vet recommended waiting 21 days between initial vaccination and doing the titer as that apparently gives time for the vaccine to kick in and the antibodies to develop. (I decided to shorten that by a week due to schedule conflicts and the fact that my guys are now double-vaccinated.)
I'd ask your vet, but mine recommended 21 days after initial vaccination, assuming they were on a clean slate pre-vaccination.