Under New York's guidelines for professional practice, which you can find at http://www.op.nysed.gov/prof/vetmed/vetpg5.htm#five11, it is considered professional misconduct to write a prescription without a timely (recent) examination of the patient to determine what is therapeutically indicated for the health and/or well-being of the animal.  Follow-up testing is encouraged not only to protect the veterinarian's professional reputation, but to protect animals from taking medications that aren't advisable or optimal for their current condition.

There’s more discussion about this topic on the AVMA website (go to https://www.avma.org/KB/Resources/Reference/Pages/VCPR.aspx.) They mention “no less frequent than 6 or 12 months” as a possible definition of timely visits.  They also point out that the term timely should be considered in light of the nature and circumstances of the patient (eg., species, condition or disease, or operation).” 

Legal Review: November 2017