Do amount of Joule mean the quantity of surges or power of one surge?
The joule rating is the maximum energy of a single voltage surge event as determined using a specific testing procedure. If the protection is is provided by a varistor, there is a correlation between the life of the varistor and the number of times that it experiences a transient that causes it to dissipate energy. However there does not seem to be a specification for that. When varistors fail, they basically explode spreading metal oxide all over if they are not shielded. If a varistor is installed inside a product, the practice is to cover it with a non-flammable insulating material to block the spread of debris. A varistor will provide protection above the maximum energy rating, but it can fail in the process. In that case, you simply throw the power strip away like you would a fuse. It has done its job and you replace it.