A good running shoe will last approximately 600-800 kilometers. If you run 50km per week, that's 3-4 months. Any walking or training around you do in the shoes also impacts their life, if you want your shoes to last longer, only use them for running. Alternating pairs of shoes can also extend their life (within that 600-800 kilometer window). This is because every time you wear your shoes, the midsole compresses. It takes 24-48 hours for the midsole to fully decompress. If it's not allowed to do so, the shoe will not cushion you as well and will wear out more quickly.