Cranberry supplements do not produce immediate effects and require consistent use to be effective. Typically it may take two months or more to observe the benefits of cranberry supplements. For this reason, it is recommended to trial a cranberry supplement for a minimum of eight weeks before determining it to be ineffective. Cranberry supplements do not produce immediate effects and require consistent use to be effective. Typically it may take two months or more to observe the benefits of cranberry supplements. For this reason, it is recommended to trial a cranberry supplement for a minimum of eight weeks before determining it to be ineffective.