As others have said, the time varies depending on the level of training - lay responder, professional rescuer, etc - and who certifies the training - AHA, Red Cross, NSC or other provider. However OSHA's new Pub 3317-05N 2006 "Fundementals of a workplace 1st Aid Program" recommends annual refresher training for CPR,AED and treatment of life threatening injuries with periodic retraining for other injuries.The pub can be downloaded at www.osha.gov.
As A 1st Aid, CPR and AED instructor I strongly recommend that anyone who has not been recertified in the last two months get recertified as soon as they can. The changes which have been made as a result of the ECC-2005 released in December of last year are significant and have a major impact on lay responder protocols. The new protocols can have a positive effect on patient survivability,as well as both short and long term outcomes. If I can be of any further help drop me a line at alindsey@fibertecinc.com
Take Care, Allan