If the lines are made of fiber glass, the roots cannot get in. If they are ceramic, the roots will continue to grow until the flow stops. What will happen next is that the discharges will back up, and if there is a concrete man hole, it will be forced up by the sewage to run free in the property, and onto the street. You will also have toilets over flowing, and odors in the bathroom and the kitchen sinks. Sometimes a telescope is used to find the roots under ground and cut them off, but that would be a temporary relief as the roots will grow back.