There isn't a set distance but most will have a set operating area they are able to cover. You should try to use your nearest fitter as this will ensure they're able to easily reach you for refits, etc.
In some countries e.g. USA you may be quite far from your nearest fitter, but do get in touch as many will take to the road booking in multiple fits/locations making their trips worthwhile.
WOW saddle fitters are independent so they decide their costs and will let you know upon your first enquiry.
