When you get sufficient profit
When you know that the price is fluctuating and going more in the forward ratio then you can set your coins for selling in order to earn profit and if the ration of downward sign is going more than you have to decide more to buy the Bitcoins or any coins you have but Bitcoin is the most responsive ones in the market. When you have a lot of money then put forward for Bitcoin and if not a lot of money then start trading and make money in order to step forward for investment.Well, where you will invest it is very important, we have many coins to invest, and some shitcoins will have more potential, and it will make you invest. Don't go for those coins. Always concentrate on top 20 coins on coinmarketcap
You are looking for big profit means you should invest in high potential coins. I will look for cheap price coins and high potential, and I have the patience to wait until the price reach my target.If you are dealing to earn more out of your Bitcoin, it could be better if you will consider to always check the value of the specific crypto-currency or bitcoin becasue there is always a right time to sell in terms of you target price and goal to earn for you to achieve something