how many hours has elapsed since the first time that patient 10855 dextrose 50% was prescribed during the current 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 = 10855 and admissions.dischtime is null ) and prescriptions.drug = 'dextrose 50%' order by prescriptions.startdate asc limit 1