There's no set rule of thumb, except that it's a good idea to buy new surge protectors after the old ones have done their job (i.e. if your area has suffered damage/abnormalities to the local power grid, serious power fluctuations, blown/failed transformers, lightning strikes, etc.)
Enweghị usoro iwu isi, ma ọ bụrụ na ọ bụ ezigbo echiche ịzụta ndị nchebe ọhụụ ọhụrụ mgbe ndị agadi mere ọrụ ha (ọ bụrụ na ógbè gị enwee mmebi / ihe nhụjuanya na mgbagwoju anya mpaghara ahụ, mgbagwoju anya ike, ndị na-agbanwe mgbanwe / , égbè ọkụ, na ihe ndị ọzọ)