With recurring uti's - my brother was given ampicillin to be followed by supportive dose of keflex indefinitely. when should he be retested?

Up to his doctor: UTI in older men usually occurs with other urinary tract problems that increase susceptibility or make it difficult or impossible to eliminate all bacteria: prostate infection, kidney stone, partial blockage, etc. Best antibiotics and follow-up testing are complex decisions, impossible to judge without more info. If any doubt, your brother might benefit from infectious disease consultation.