how many hours has it been since the last time during this hospital encounter that erythromycin was prescribed to patient 49176?
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 = 49176 and admissions.dischtime is null ) and prescriptions.drug = 'erythromycin' order by prescriptions.startdate desc limit 1