The Strings and cables on a crossbow wear easily and fast over time from all the outdoor exposure. It’s best to replace the strings and cables as soon as you notice any signs of breakdown because when they break, it’s no pleasant.
To help prevent the the breaks on the string you will want to apply wax liberally to the entire length of the string and cables, making sure to stop right above the shooting rail. Wax on the shooting rail could clog up the trigger mechanism.