how many hours has it passed since patient 2518's first prescription of bupropion during their current hospital encounter?
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 = 2518 and admissions.dischtime is null ) and prescriptions.drug = 'bupropion' order by prescriptions.startdate asc limit 1