If about 75% of the top 50 coins performed better than BTC over the last 90 days then you’re sure you are in Altcoin season.
When you have that number that is draw from performances of 50 top altcoins in the last 90 days, you're late. 90 days mean 3 months which is a long time and when you see 75% number to be sure that it's an altcoin season, you buy in, and you will stuck in a correction or ending of an altcoin season.
One thing to remember, when Bitcoin reaches to its all time high, altcoins won't have too long time like 3 months for their altcoin season and if you only participate in the altcoin season party after 3 months, you will like buy at peak of altcoin season.
It's a good topic and reminder. It's my first bull run in which I have invested some money, and now I was also looking to invest in alts but did not have much knowledge about the idea of how I can know when an alt season is coming, and here you just shared it. Haha, it's like I was having a question and coincidentally you just answered it.
The best time to buy altcoins is months like a half of year after Bitcoin finishes its bear market. After a half of year or 1 year since a bottom of bear market, what are worst already appeared and you will have less risk of black swan events to kill top altcoins. See examples with Terra and FTX, 1 year after these fiascos, things were better and no more black swan event.
Buying top altcoins 1 year after the bear market and hold them, don't wait for 75% figure as a confirmation of altcoin season to join.