I have never been a wax and tune guy. Sure I get them done yearly but I always figured I wasn't racing the world cup or working at a resort. So for the avg skiing year less than 20 days on smaller hills. My tune should hold up for the year.
Valid or not.

How long should a tune hold?

Depends. A lot of east coast ice will dull an edge quickly. I usually summerize, then do a mid year tune after 10 days or so. Some people will tune as frequently as 3 - 5 days, but I am not core enough for that kind of $$$.

Here's my take. And I have to say my parameters are probably shaped by being a pilot, which does affect your mind. I consider my skis as a life support equipment. Same as brakes for the car. I don't go on a budget when it comes to maintaining the skis.

1. I do a tension check and full pre-tune every year.
2. After every ski day, I dry and use the liquid wax on the skis and check for damage especially the edges or any Ptex gouges that may interfere with skiing.
3. Every ski week I use the ski iron and wax and sharpen the skis.
4. After the season is over, I do a full apre-season tune which may entail machine grinding and Ptex repair.
5. My skis never ever see the storage room or garage. They are in my closet, a spotlessly clean place with low humidity. They go on a ski hanger, not on their side or against a wall as that may affect the camber later on.
6. I do a tune pre-season also.
7. Boots are also part of the life support equipment. I put them in a breathable boot bag so I don't end up with dust or, unlikely in my closet, little spiders making their home in the toes.

Having said that, I was aghast at the drastic drop in DIN settings because of one day in my life and may choose to set my own DIN in the future. I'll give it a try though.

I'm probably OCD on taking care of my skis, but on the other hand, they last like my cars. And usually, after every two or three years, I give a perfectly good - and safe- pair of skis to one of my friends.

The edges -should- hold up to a -reasonable- sharpness for 20 days, but only if you're not a hacker. Figure maybe 4-5 6-7hour days if you are before you definitely notice lack of grip on hardpack.

The wax should hold up up to 3 6-7hour days around here depending on snow quality, more than that and base damage is quite possible particularly the area next to the edges, and particularly near the binding.

I have to wax every 3 days or so when skiing on manmade snow; I let the bases tell me when (ie, look for dry spots). At the same time, I sharpen the edges as needed. With this regular/minimal tuning, I rarely have to do a serious tune. That may only happen once a season. I never get shop tunes anymore; I found that they can't do as good of a finesse tune as me, and many shops are heavy-handed with the grinder, which can dramatically shorten the life of a ski.