Is most advice only helpful for a while until too many people follow it?

I think you can put advice into two buckets. You've got core tenets you need to follow, then there are time sensitive advice, which can only work before it's popular, or can only apply to certain time periods for other reasons.

"You need a good hook", "you need to focus on a target market" is good marketing advice that even if everyone follows it, will still be good advice.

"You need to be on tiktok/insta/youtube" is only good advice before too many people start doing it, and you just become part of the noise.

"You need to advertise in the yellow pages" is only good advice until people stop using the yellow pages. Different from there being too many other marketers doing it.

It depends on the intent of the advice.

If the advice is intended to let you "win the game", or "get ahead of the competition", then yes. If everyone is doing the same thing, then using it isn't going to provide you with any particular advantage over anyone else. "Add these words to get to the top of Google results" is doomed to eventual failure.

If the advice is intended to help you improve your quality of life, or improve at something specifically for the joy of being better at it, then being well known will not dilute it. "If you're feeling sick, then you should drink more water and rest" is just as valid now as it was when it was a new and revolutionary insight.

There's always a lifespan on information, including advice or other forms. For example, shouting "Fire! Get out!" - the advice is very good for that moment - if it's true.

With business and marketing advice the timing is mostly susceptible to market cycles, which for a lot of fields are generally in the 18-months to 4-years range; by the time 4 years have passed, a whole new cohort has completed college education, the political landscape has shifted and a lot of people have made some change in their lives - the party is over and you have to find new opportunities to hit big. It's not just that too many other people follow that advice, it's that it has become a competitive field in its own way, and often subject to forces that favor an incumbent. So you can't actually get in, or you'll experience headwinds that push you down the ladder. This is true with specific products and services, with career paths, and with categories of assets.

The very best advice is just general wisdom, proverbs, old life philosophies, etc. It needs study and interpretation to feel applicable to current events, but if used well it can reveal an opportunity early on.

There is fad advice and longer term solid advice. I know more about sales than marketing, but for example having a targeted video pitch rather than a cold email is probably a gimmick that if one person does it it's more likely to get interaction but if everyone does it its annoying. Talking to your customers and understanding their problems is advice that will continue to work, even if everyone does it.

In many cases, I believe it is. Consider Tim Ferriss' book 'The 4 hour work week' for example. It had lots of creative ideas, and many folks used them successfully. But by the time the book was published, the market had changed. Google Adwords used to be Hella cheap, which is part of what enabled methods described in the book. But they're not anymore, and there's too much of everything.

I think part of it too, is that over time the advice gets watered down, and this watered down version is then taught to further "generations".

Going back to the origin and then extrapolating to current conditions can probably yield great results

Coined the term "reflection completeness" for precisely this: https://sdrinf.com/reflection-completeness :)

When your barber starts recommending stocks to you, it is time to sell.

Unless of course everyone starts listening to me.

Absolutely. If too many people do one thing. Its no longer useful advice and see as obvious.