It varies with the MT and with the dictator. Very experienced MTs might take only 1.5 hours. Others might take 2-3 hours. New MTs might take more, and student MTs might take . . . days.
If there is a lot of material in the 1 hour's worth of dictation, then it will take longer. If there is a lot of dead air, it might be possible to transcribe it in an hour flat.