That varies for every household. I find the best way would be to consider it's present performance, if a knife struggles to cut tomatoes or peppers, through the outside of the skin in then that knife is in need of sharpening, it may be held of a little longer but if you have 3 out of 5 in this state then I would advise getting them sharpened straight away.
I'm sorry no, I only provide a sharpening service for Tct blades, however I do my best to repair damaged teeth but any that are missing or heavily damaged I cannot help with.
I may do so in the future but at present no.