I don't know if I believe teams on the World Cup are grinding skis, doing a short prep (1 day) and then racing the next day on the World Cup at -18C (or -16C).
In the last three seasons, there haven't been many days that cold on the World Cup. Maybe close to that in Rybinsk in 2015 for the 10/15km skate, maybe in Ruka in 2012, maybe 2013. But if there were, and we'd just gotten a pair of new/newly ground skis, we'd get them up to "speed" in less than a few hours.
If it was crazy cold, maybe we'd throw a couple of layers of cold CH/LF on them before going to paraffin, but in three years, I've only done that maybe six times. It's not a huge thing - but that's also assuming that you're going to be putting on a "race" base, then a "race" HF paraffin, then a powder, then a topping. And that's also assuming that the skis you're testing have the right grind for cold conditions, and have the right camber profile for cold conditions; good skis and grinds are pretty forgiving.
As for prep on a new (or newly ground pair of skis): Generally, the skis show up from the grinder (or rep) with some soft, red wax on them. It's often rolled on from a pan, not ironed, so I don't think it's penetrating too deep. So that gets promptly scraped off. Then we'll put on a layer of CH6 at ~138-140 C. Maybe iron it twice. If there's time: Scrape, brush, re-apply CH6. No hot boxing.
From there, that's considered ready to go into race waxing. Like Randy said: A base paraffin (usually LF), and then on to HF paraffin, powder, structure, topping.
Anyway, if you got a new ski/grind, we're talking about 20 minutes of work on a particular pair of skis to get them ready to be raced on (though that work is done between doing a ton of other skis - so 20 minutes of actual work on a particular pair of skis, within a six- to 10-hour window of working on stuff).
For example, prep on two pairs of skis that have won medals:
At the Team sprint from Val di Fiemme 2013, a pair arrived on Saturday afternoon and was raced to a win Sunday morning - so maybe five layers of paraffin total - two layers of LF6, an base paraffin, an HF to test, another HF of race wax (because the snow was dirty). (Plus powder and many applications of topping between rounds, but I don't think of pure fluoro as conditioning a base).
For the Team Sprint from Falun 2015, a pair showed up that morning - definitely, only five layers of paraffin (same as above) before they were raced (and they were really fast, though the result doesn't reflect that). That same pair was used in the 10km, and after it was fluoro cleaned Sunday night, it got a layer of CH6. Monday afternoon, they got tested with an HF on them, them cleaned and covered with CH6 for the night. Tuesday morning, a base paraffin, an HF to test, and (again, because it was dirty), the HF race wax. So that's a total of 11 paraffin layers: 4 X CH6, 2 X base paraffin, 5 X HF.
As a side-note, we used to store skis in LF6, but two seasons ago, we ran out, so we asked Norway if we could borrow some, and they told us they were using CH rather than LF. So we changed, too - it's much cheaper. And the main reason we're using Swix is that it's so readily available: You can get CH6 in any shop, in a pinch. There's nothing special about CH6; it could be Rex Blue or Maplus Red or Toko Red or Holmenkol Beta Red or whatever.
They are grinding between just about every race on the WC. The fastest skis are those with fresh base material.
No, not grinding that often. Fresh base materials can be very fast, true, but a) not always and b) logistically, getting skis to a grinder and back to the next race venue is an issue. A handful of teams may be all running on National Team grinds (e.g., Norway, though they don't regularly travel with a grinder), but many, many others are using grinds provided by ski companies, along with a small handful of proven grinds from shops/techs. I work with a fleet of 28 skate skis and 17 classic skis, and there is one pair of skis that does not have a "factory" grind.
The ski companies will re-grind, but of course, the same issues arise: They don't have a grinder with them, so the skis need to go back to Austria/France. Sometimes that can happen over the week (if we're close to those countries, and the rep is driving home for a few days between races), but usually not. I've had ~8 pairs of skis re-ground in three years, though there are a few pairs in the quiver that for sure will need to be freshened up soon.
One thing we do that I believe helps reduce the need to frequently grind is to religiously fluoro clean any ski that's been powdered or topped. That, and to be careful with the speed of the iron, so we're not thermo-sealing the bases. But the skis definitely "absorb" wax better after they've been cleaned, and you're not just ironing a bunch of pollution into the base.