My experience is that the 998 certificate is only valid for 4 months, however, most USDA vets are familiar with the APHIS 7001 form, which is the international health certificate for export/import that the US government uses. This document is only valid 10 days or less before traveling, therefore I believe that the USDA sometimes (depends on which office you work with) applies this standard (10 days before travel) to the 998 form as well. Your airline may require both the APHIS 7001 and the 998 to accept your pet (In my experience, Continental did).
The USDA office I went through (in Annapolis, MD - district office for MD, DC, and VA) also used the individual id number on the APHIS 7001 form to fill out the "number" section of the EU 998 form (up top - says something about certifying official number or something, I don't have it in front of me). Therefore, they wouldn't complete the 998 until I was within 10 days of traveling, so that they could fill out the APHIS 7001 and use that forms' id number to complete that field. Again, this can vary by USDA office.
One note - the first official vet section is your vet - ensure that they complete that section when appropriate. The USDA vet only fills out the competent authority section (I made this error with my own paperwork, thinking that the USDA vet was both the official vet and the competent authority.)