You really only have to worry when the snake starts losing weight, so I'd say just keep offering and monitoring weight until you think it's starting to become detrimental to your snake's health. Generally once they get hungry enough (and if the f/t prey is hot enough - hair dryers work wonders!) they should accept it. If you have any live rats handy, you could even try rubbing the f/t one on the live one to scent it.