how many times is the nafcillin desensitization since 68 months ago prescribed to patient 17694?
select count(*) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 17694 ) and prescriptions.drug = 'nafcillin desensitization' and datetime(prescriptions.startdate) >= datetime(current_time,'-68 month')