What is the required waiting period for a woman whose menstrual cycle is habitually prolonged?

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If a woman has a customary habit of prolonged separation between her menstrual periods, her waiting period ('iddah) is not completed until she has experienced three menstrual cycles, regardless of how long the interval may be. This is because her menses has not ceased entirely, nor has she exceeded her usual timing. She remains classified as one of those subject to fixed periods (dawat al-quru') and maintains her established cycle, resembling a woman whose periods do not widely separate. There is no known disagreement concerning this ruling.