Don't know, guess it depends on how often you use it. I would talk to the guy that does the sharpening he maybe able to give you some tips on how often you should sharpen them. Then agian its going to depend on the quality of your knife steel if they are good blades they should not need it has often. I have several chef knifes that I have never sharpened and they are still razor sharp. As far as the costs it does not sound like a lot to keep one of your most important tools ready when you are.
Mad Sugar Artists Society making flaming tracks!!
I use S.S. Dexter Russell knives wich are pretty tough to put an edge on. I use a 2" X 6" "meduim" diamond stone and it puts an edge on these knives in no-time. These diamond stones are unbelievable! Like magic.
I like the diamond stones as well Gorgonzola and have a 1000 grit one. Here's a link to the way you can sharpen to the Japanese style. I do my knives this way now and found they work better for me, you can sharpen left or right handed. Perhaps try with an old knife first.