I think it depends on the individual and what your goals are. On the one extreme are people who are happy skiing as they ski. They really never need a lesson once they've learned to be safe if they don't feel inclined to improve. On the other extreme are racers, who receive coaching on a daily basis (arguably different from lessons). Most people fall somewhere in between.
I'd recommend new skiers take lessons nearly every day out for at least the first five or six outings if they want to maximize their development. New skiers require directive external feedback because they haven't developed the awareness necessary to improve independently. They also need continuous feedback while developing muscle memory so that the patterns they learn are effective (or at least not detrimental). New skiers often see some or significant regression between sessions without coaching. This is where you'll see giant leaps through regular lessons. Skiers who have lots of ingrained issues at the intermediate level often didn't get much quality coaching at the beginner level.
Skiers at an intermediate level can typically demonstrate consistent performances on easy snow conditions and terrain. They're less likely to regress between lessons and can start improving their skiing by working on specific skills through targeted activities between lessons. I like watching intermediates progress as they still have lots of "breakthroughs" every few days on snow. I see good progress in intermediates who take lessons every few days. Other intermediates plateau even with lessons for lots of reasons.
More advanced skiers who've had lots of lessons in the past (especially instructor types) are well aware of their issues and typically have strategies to continue development. These guys can continue to develop with only a few lessons each season. Development is gradual here and you reach a point where lessons leave you saying "ok, nothing new here" more often than not. Also, at this level, sometimes the body needs time to consolidate several skill improvements together to push your skiing ahead--especially if you can temporarily demonstrate a new level of skill and then see it disappear again.
Experts... To me, only skiers who ski at the ability of level four instructors qualify as experts. I can't personally speak to this ability level. However, I'll ask one of my coaches next time we speak.
Many racers receive coaching (which is arguably different from instruction) nearly every ski day.
Everyone progresses at a different speed even with lessons, and gets different amounts out of each lesson. Personally, around 90 of my first 100 ski days were in lessons, then maybe 10 of my next 40. This year I've had lessons maybe 20 of my 60 days on-snow so far, but eight of those were for certifications--so really only 20% of my days were lessons. Also, I was in a ski club for my first 100ish days and got free lessons, so I'm an exception and a feedback junkie.
I'd suggest a bit more time between lessons as you improve in these areas:
- Mind-body connection
- Familiarity with the sensations of skiing
- Skiing theory knowledge
- Self-assessment and self-development skills
- Comfort on desired terrain
- Number of instructor friends ;)
Some good indicators to take a lesson:
- Your personality type responds well to external feedback and encouragement
- You feel you've hit a plateau
- You have specific challenges on a terrain or snow condition
- You need inspiration
- You can see elements in others' skiing that are lacking in your own without knowing how to progress
And during/after a lesson, to maximize the value you're getting, you can:
- Make notes on the chairlift or at the end of the day
- Consider what instructors say with an open but critical mind
- Ask questions during the lesson, especially on the chair
- Every run, think about one keyword related to an improvement you're trying to make (e.g. "steer" or "pivot" or "center")
- In your free time, alternate between a free run, then an exercise to work on an aspect of your skiing, then working the exercise back into your skiing
- Keep reading these forums :)
Whew--sorry for the long message, but I hope it helps!