how many hours has passed since the last sodium polystyrene sulfonate prescription of patient 99647 in this hospital visit?
select 24 * ( strftime('%j',current_time) - strftime('%j',prescriptions.startdate) ) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 99647 and admissions.dischtime is null ) and prescriptions.drug = 'sodium polystyrene sulfonate' order by prescriptions.startdate desc limit 1