We're planning a trip for next July (flexible on dates) and plan to use Amex Membership Points transferred to Delta to get 4 first class seats. We haven't decided yet if we'll go Houston to St. Thomas on Delta or take Delta to San Juan and pay for the hop to EIS.

I'm looking for advice as to the optimum time to acquire the tickets. Does the mileage required for a particular date typically change or does it stay fixed? One issue is of course that we don't want to jeopardize our chances of finding seats by waiting too long.