The answer can be very subjective, based on the level of skiing and for their desire to really get to know a place rope-to-rope and find all the hidden stashes.
For a medium/large resort, it probably takes a whole season to get to really know the place. It isn't necessarily based on the number of visits, but really getting to know how to "read" the mountain and conditions. That usually takes skiing the mountain under a variety of conditions, at different times during the season, and remembering the good/bad/ugly of your experiences. Observing other skiers and skiing with different people also can really help to get to know a place.
Specifically, to understand:
- Snow conditions on different aspects (direction, sun angles, which areas get sun-baked, which areas corn-up best)
- Weather effects on different areas of the mountain (wind loading, areas that generally receive more snow)
- Skier patterns / flow
- Lift lines (what lifts are crowded/uncrowded at different times of the day)
- Grooming patterns (e.,g when do they periodically groom that black bomber run, about once every two weeks)
- Well know "secret stashes"
- Your own "secret stashes"
- Observing other skiers and where they go/don't go
- Skiing with different groups of friends who know different lines/aspects, or just ski the mountain differently that you normally do
- Understanding good early/late season area
- Secret cut-offs (to avoid traverses, cut to other lifts, shorten hikes)
- Understanding when/if patrol will open certain terrain (e.g, opens typically around 11am after a storm, or never opens for a few days)
- Understanding which lifts are likely to be on wind hold
- Knowing days to avoid the resort all together (weather, crowds, conditions)
- Knowing which other nearby resort to ski instead of said resort based on condition)
That being said, I've been skiing some places for over 20 years. I know the in's & outs of Vail, however I still always learn a thing or two when I ski with different people.
Snow can also have a big impact. There a few lines and stashes at Vail, Breck, or A-Basin that have only been skiable about 1-2 seasons out of the past 20.
It was interesting to see how the dynamics of Vail changed when they opened Blue Sky Basin about 10 years ago. Same thing at Breck with Peak 7, the Gondi, and Imperial coming on-line over the past decade. Also, there are some places at each resort that used to be a "popular" stash years ago, now that rarely get skied.
I've done the one & done resort hopping - I enjoy exploring a new area but you can also be very inefficient in your skiing or miss out on the good stuff without research. I had a lousy experience at the Canyons just for that reason.