I personally don't believe that there is a number - it is down to the quality of the nails - I don't think you ever need to soak off just for the sake of it. If you use a product which will show signs of age, cracking, yellowing etc. then you probably will need to soak off and replace. Sometimes you will get a client who will choose to have a new set - maybe she will not like to see her own nails growing up behind the product.
I personally think that if techs are soaking off just because, say the smile line needs replaced and they only do white tips, then they need to improve their skills rather than make their clients pay the extra.
I think the ultimate for most clients would be to eventually have their own nails (ie. without tips) with a L&P or gel overlay.