How long your wax lasts is all about how durable your wax is, and durability comes from both; 1) the wax adhering to the base and 2) the wax being hard enough to resist the abrasion from the snow crystals. The colder and more aggressive the snow, the harder the glide wax needs to be, but hard waxes do not adhere well to the base, so to only wax hard waxes will result in low durability.
Most often a shop will provide a seemingly low cost "wax service", but only apply a low-grade wax with a Wax Jet. Wax Jets are machines that have two rollers, one heated fibertex roller that the tech
will apply (or rub) the wax on to before running the skis over, which applies a thin surface coat of wax to the base, and a second brush-like roller that polishes the wax. It takes just a minute to wax a pair of skis with a wax jet, however, this application method does not provide for a "binder" (a penetrating prep wax) that anchors the glide wax to the base. Without this anchor the wax is easily scrubbed away by the aggressive snow crystals and your results will be mediocre as the durability of the wax job is very low. In essence, you are paying for someone to polish your skis with wax rather than having it applied with an iron so it absorbs into the base.
Many shops offer a "hand" wax, which usually means they will use an iron to apply the waxes resulting in a much more durable finish and, if you are not going to hot wax yourself, it is recommended you specifically request a hot-iron applied wax. For maximum wax durability, it is best to first apply a penetrating base prep wax -- which is a soft wax that absorbs into the base -- prior to applying the glide wax. Glide wax is a harder, more abrasion resistant wax that will adhere well to the base prep (aka-binder). Many shops will apply base prep before the glide wax if you make the specific request, it might coast a bit more, but the end result will be much better.
The saying goes; soft wax penetrates and "holds on" to the base better than hard wax while hard wax resists abrasion better than soft wax; and hard wax adheres better to soft wax than it does to the base. So for maximum durability first iron in base prep (allow to cool and scrape) then iron in your glide wax (allow to cool and scrape), now your wax job will last much, much longer.
For more specific details on wax application and, while there, look for the Renew Base Prep waxes as the use of this product is the key to making your wax job last.
BTW, even a well waxed ski, saturated with base prep and coated with a quality glide wax (like Zoom or Zoom graphite), will not last 10 -12 days of aggressive skiing (that's crazy talk).
My rule of thumb is to hot wax every two or three full days of skiing.