we have nothing to say concerning the best time to buy altcoins or any coins, what I understand in this is that coins have to do with the kind of project it's, because when you not know the quality of the project when you invest you might experienced loss, investors doesn't follow a trending coin to invest, any investors that follows that might end up losing its capital, because some of the altcoins might die off on the process of long-term and that it's advisable to make proper investigation towards coin before you can invest on it.
Although we cannot say the best time to buy any coin, we can say that the best time to buy altcoins is when the price of Bitcoin starts to rise, i.e. the bullish season begins. Because if we buy altcoins when the price of bitcoin increases, then the price of those altcoins will increase with the increase in the price of bitcoin. Moreover those who invest in altcoins should check the quality of the project before investing as investing in altcoins involves maximum risk.
There are many alt coins which can earn good profit by long term investment like Ethereum, BNB. Among the altcoins, I vouch for holding these two coins for the long term. We saw BNB below $300 when the price of Bitcoin was below $50000 before it touched an all-time high. BNB coin price crossed $500 when the price of bitcoin rose high so long term investment in any coin or potential altcoin is guaranteed.