What effect have tides had on the length of a day

Tides do place a drag on the earth's rotation, but gradually. This energy lost from the earth's rotation is picked up by the moon's orbit around the earth. The moon is receding from the earth. So, in answering your question, the length of the day has been increasing. Without getting into the arduous math involved, in the very distant future the moon will have reached its max distance and the earth's rotation will be in step with the moon's orbit.