28 CFR 549.61 - Definition.
As defined in this rule, an inmate is on a hunger strike:
(a) When he or she communicates that fact to staff and is observed by staff to be refraining from eating for a period of time, ordinarily in excess of 72 hours; or
(b) When staff observe the inmate to be refraining from eating for a period in excess of 72 hours. When staff consider it prudent to do so, a referral for medical evaluation may be made without waiting 72 hours.