In general, the right time for investment when prices are low, and not only that of course depends on you too, because investment also depends on the capital you have, large and small capital is one of the important things to determine where you will invest.
It is true, investing the best is when prices are down, because prices in the market are unstable and will lead to ups and downs, so when you see prices dropping, you can make purchases.
But now very many Altcoins are in low-cost positions. You need to choose those coins where high liquidity and lower capitalization.