So you have to be a complete idiot to but stock in a company with a P/E of 500!
This is obviously untrue. Would you sell a box that spits out $1 million dollars a year for 1 million dollars?
This calculation is why "growth" companies dominated the stock market during the 2010's: with the Zero Interest Rate Policy that most of the developed world had, the discount rate that the markets used ended up being basically zero. In which case a market player is indifferent between a dollar in 2020 and a dollar in 2040. So if a company had a 10% chance of being worth a trillion dollars in 2040, that was worth (0.1 * 1 trillion=10 billion dollars). But with a more traditional 4% discount rate then a dollar in 2040 is worth less than half of a dollar in 2020, and that means your 10% chance of being worth a trillion dollars in 2040 has less than half of the value. Even if nothing else changed about your business, just the discount rate changing halved the value of your company.
The earnings period is 1 year.
It would mean making 100% return on investment each year. Being that low is only possible if there's reason to think the business is extremely precarious and unlikely to survive.
P/E 30 means returns of 3.33%, P/E of 20 means 5%. These are sensible numbers given people have other investment opportunities.
P/E of Tesla being 400 or so means it would take 400 years of its own profits to be able to afford to privatise itself, i.e. returns of 0.25%; being that high is a gamble that future revenue/unit time will go up by a factor of about 20 to bring it into the sensible range.
The upper bound from the grandparent comment for P/E 500-1000, says the annual return is 0.1%, which is what I saw on various current accounts, not savings accounts, not special deals, current accounts.