how many hours ago was the last time patient 12927 prescribed ns 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 = 12927 and admissions.dischtime is null ) and prescriptions.drug = 'ns' order by prescriptions.startdate desc limit 1