I think neither of our understandings is exactly correct. There appear to be two deadlines. This is the original FAA Notice:
http://www.faa.gov/documentLibrary/media/Notice/N_8900.274.pdf
First deadline is scheduling the exam:
The letter will notify the airman of the decision to reexamine and require the airman to contact the Fresno FSDO POC (referenced in the letter) within 10 business days of receipt of the letter to schedule a reexamination.
The second deadline is the reexamination itself:
When the Fresno FSDO is contacted by the airman, they will allow the airman to choose from available dates and times to schedule the reexamination. The reexamination should normally take place within 15 days of receipt of the letter of notification; however, there may be exceptions that delay the reexamination in accordance with
FAA Order 8900.1, Volume 5, Chapter 7, Section 1, subparagraph 5-1419C. The inspector should accommodate the airman as deemed necessary and appropriate to the situation, but may not allow the airman to postpone the reexamination indefinitely. If the airman does not intend to operate aircraft for a short period of time, or needs time to practice and cannot meet the 15-day deadline, the airman may temporarily deposit their airman certificate at the FSDO for a maximum of 30 days in accordance with the temporary deposit and reissue policy (refer to
FAA Order 8900.1 Volume 5, Chapter 2, Section 5, Miscellaneous Part 61 Certification Information).