There's no hard and fast as tecman says. If it's a solo flight that is a more or less 'must do' (OK I know there's no such thing as a 'must do' in GA) like a servicing flight then if I was slightly under the weather I would make that call and do whatever my call was. If it's taking two non pilot friends to France for the day then other than fighting fit and on the ball is not an option.