If it switches more current than it was designed for, zero.
The question can't be answered without knowing the load. Spec sheet ratings are generally given for purely resistive loads. Inductive loads will reduce the life. Suppression will mitigate the impact of inductive loads.
If you are switching an output so frequently that relay life becomes a concern, you're probably better off with a solid-state output device. If, for whatever reason you really need the contact output, consider a solid state output driving an interposing relay so that when the relay fails it is easy to replace.
If you need one more output and all you have available is a contact output, you can still add an interposing relay. That will give you a relatively small load on the PLC output and the interposing relay will be the more likely point of failure.