My vacation record is 28 days straight. I've also done several 2-3 week trips where I've either skied every day or only had down days for travel between resorts. For a few trips I cheated by skiing one day and riding the next. My normal routine to prep for these trips was a 12 week program of 3-4 workouts per week mixing strength and cardio, plus fun stuff 1-2 times/week totaling 12-15 hours.
I'd be leery of any minimum level of fitness (e.g. run x minutes per mile, lift x% of body weight, have x% body fat). There are too many resort specific factors (e.g. altitude, slope pitch, snow quality) and individual differences (e.g. personal tolerances, skiing styles, attitudes, goals) for a specific guideline to apply.
It's also been my observation that the average skier needs a day off every 4 days. My suspicion is that such skiers would either maintain that pace over a longer trip or vary the intensity of their skiing or include some half days. Very few "average" skiers are going to spend the time necessary to get fit enough to go full bore for 2 weeks.
It's been my experience that most mountains out West have the capability to suck the life out of anyone. No matter what your skill level or fitness level is you can set the timing and intensity of your skiing to create excessive fatigue. More advanced skiers have more options to conserve energy without getting bored but they also have more options to burn more energy than lesser skilled skiers (typically getting more of a total body workout versus one particular worn out body part). For example, an intermediate skier is much more likely to get sore quads and need a break day than an advanced skier. And no matter how much that intermediate skier works out in the gym, the mountain is going to burn their quads before 2 weeks is up. For the average recreational skier, a one hour lesson is worth at least 10 hours in the gym.
For those of us serious skiers who previously would take 2-3 one week trips per year and are now considering combining them to reduce airfare overhead, a slight increase in pre-trip conditioning, combined with smart opening trip acclimatization and healthy during trip behaviors (moderate alcohol consumption, plenty of sleep, proper hydration and diet management) combined with reasonable daily choices of intensity and length of day (e.g. not skipping lunch and doing mogul laps from first to last chair) will allow for a successful trip without having to "waste" days off slope.