I go by the overall look and health of the animal. I don't think it's as simple as just setting a date. I had a set of twins this year that refused for several weeks...I assist fed them several times and they are eating like champs now. On the other hand I have a snake that has not eaten yet and it's been over a month. Her weight and overall health seems to be fine, so I'm not stressing over it yet...Just keeping an eye on her and offering her meals every week. I think you just have to use your personal judgement.