There is no particular time to invest in bitcoins and eth or whatever coins it is. As long as you have coins to invest, you can invest it immediately. Holding your coins in you wallets is also a form of investing, because your saving it to gain profit out of it. Values of coin can either increase or decrease you always need to take risk. Just hold it as long as you can and do not sell in it is not necessary. Having an investment especially bitcoins can make anybody rich.