As soon as you'd like. HotZippy has no official ruling at this time to what constitutes an ineffectiveness or over-saturation of market value. If we do see signs of such fatigue we may ask the author to hold back for a certain amount of time. This has not yet happened during our years in business but may happen in the future. When that occurs an official ruling will be added the Advertiser Terms & Conditions. Until that time, there is no limit for often a book or an author may be promoted by us.