when you pay good money for good shears it only makes sense to keep them sharpened and in good working order, so they last a life time. just like a really good butchers knife, with time and use they can dull.there is no reason to not take care of them and have them sharpened, if you don't they just become...disposable...and i spent way too much money on mine to throw them out when they get dull.
as for the once you do it....
well, doesn't that make sense? after you have them sharpened, if you use them...they will at some point dull again...it is the nature of the beast. if you do not sharpen them they loose their usefulness, if you do, then yes you may have to again some day...it's not like you sharpen them once and then they are always dull, or you sharpen them once and never have to again. it is relative to how much you use them and for what.