Just to attempt to reel this in a bit, the original poster stated he skis 20 to 30 days (which is above average, I think), and doesn't currently do anything much, if at all to his skis. Quibbling over what kind of structure the bases should have and when or if to change it seems extreme!
I've read that approximately 97% of the skiing community never waxes. They ski on what they got when they bought the skis.
Original question: " How often do you need to have your skis tuned?"
Short answer - every time! By this I mean, keep
them in tune by waxing every time; learn what to look at; check hte bases and edges. In my view, bringing skis to a shop for "tune" is a major overhaul, not routine tuning and maintenance.
I don't trust just any shop to "tune" my skis, as an unkonw shop is just as likely to de-tune or ruin them. Even if the do it correctly, you really don't want a base grind mid-season unless the skis are seriously messed up. For example, f there are vast white-colored patches on the bases ("burnt" bases), you need a grind.
My recommendation for someone which isn't obsessed with ski tuning is care and maintenance, which includes waxing every time.Care
: wipe your skis off after you're done, and store them dry. Even better, rub some wax on the edges to prevent rust, after wiping them off. If they went from cold to hot, like putting them in the basement, check them or wipe them again after they come up to storage temperature, as condensation will wet them. Rust can pit the edges, and also, pitting at the edge/p-tex interface can ruin them.
If you know or suspect that the spring snow was salted or otherwise chemically treated, wash the skis and bindings and boots thoroughly, just as you would your car, and dry them well for storage. Check that the bindings are dry. Mild car washing soap won't wash out the binding lube.Maintenance
: If you do nothing else, touch the base, touch the edges and wax. By "touch," I mean very lightly remove any feature that exceeds the original as-tuned profile. Examples are parts of the p-tex base that stick up as a result of gouges or other wear or damage; filing or otherwise dressing down burrs in the edges with a stone. Unless just dressing down edge burrs very locally, that is, if you're going to touch the whole edge, get a guide so you don't unintentionally dull or round the edges, or mess up the base and side bevel angles. A good guide is the adjustable one from FK-tools http://www.fktools-us.com/
. To do this, you must know what the angles are. A good shop can tell you. The manufacturer can also tell you. To wax, the least cost, safest way is to rub the bases with the wax, like a crayon, and rub it in with a cork. The rubbing creates friction which melts the wax into the base. Rub in a lot if its the first time, doing so repeatedly. A really effective and fast "cork" is a device made by http://www.alpineskituning.com/
Ray's Way, waxWhizard. Its a cylinder with woven nylon wrapped around it.
If you only want or need one wax, get the red stuff, or Swix F4 or some other general purpose wax.The Next Level
: If you start to care about enhancing performance through tuning and waxing, get a brush or two. First, get the nylon. The second should be bronze. The brush is mostly for brushing wax out of the structure of the base, both before new wax, and also after waxing, to expose the structure for better gliding, and no suction from surface tension.
If you store your skis dry, kill the burrs, and wax often, you are far ahead of average and your skis will work better and last longer, and will ski easier and faster.
Beyond this I'd recommend finding someone nearby who will help you out by hot waxing and scraping, and hot-boxing your skis for you, to get rid of the dirt and "hairs" on the base, and load the bases up with wax.