If I get a ski waxed but don’t use it for 2 weeks, does the effectiveness of the wax wear off? If there is degradation, how long does a waxing effectiveness last before you would need to wax again if you didn’t use it?
Generally what you MIGHT see is some hazing as the wax works its way out of the structure wth temperature fluctuations. A quick brushing with a bronze or steel brush will have it all shiny again, but unless your first push off is through timing wands you won't notice a thing if you don't do that.
Two weeks? I don't think it will change anything. The wax itself will be fine but other things such as dirt or other particles may accumulate on the ski. As Sibhusky said, give it a brushing before you head out to get any "dry" wax and other dirt off of the base. It should be fine.
Fresh snow, old snow, spring snow, artificial snow... they all have different crystals and properties and requires different waxes. So even in two weeks, if you're skiing in an area it hasn't snowed for a while but you're still using your fresh pow-pow HF wax, it won't be as effective as an LF wax which is optimized for the snow crystal structure and moisture content of older snow.
If you're wondering about long terms, it's common for world cup race techs to wax skis throughout the summer multiple teams even when not on snow. This gives "juice" to the base to better dissipate water particles as best as possible when the race season does come again. A recent podcast on Blister's Gear 30 pocast talked to a Head representative about waxing, skis, and constructions. They are also competing for tenths of a second so for us mere mortals, that level of commitment does nothing.