Although Bitcoin and others some altcoin already higher than previous ATH, that not mean the altcoin season will end. We have to analysis it according to the total of market cap that growing, the increased of institutional investor join in to crypto and the mass adoption keep stronger. From my personal analysis, the altcoin season will extended until in the beginning of next year.
Quite agree with your opinion on the time frame, but i think the altcoin season will last for 1-2 months only. We might see a a long accumulation zone for 3-4 months before altcoin season. In these months, we should look for the project that focus on build their products because their coins/tokens might become the best performance in the altcoin season.