Monkey bar rungs are typically made from solid steel, galvanized steel, wood, cable lines, and rope. Their durability depends on the materials from which they're made. For example, rope monkey bar rungs won't last as long as galvanized steel outside because it will begin breaking down in the elements. However, if each of these materials is used indoors, they're longevity will be much longer.
The rope and cable lines will wear out sooner than the wood and galvanized steel from repeated use, which is one of the drawbacks of these materials. However, wood and galvanized steel require more maintenance in comparison to those materials to increase their longevity. So, there's a balancing act to consider when looking at these materials and making your choices.