It depends on your strategy for me I don't wait until 20% for me to take profit, most times, some coins hardly make such gain, but taking profits like 5% or 10% many times could be a good idea, coins normally makes such gain many times before it could go as high as 20%, so I believe in taking profits as soon as you see them and re buy when it dips down again.
You should also know that you stand to miss out on huge gains if you sell too soon. I know a couple of guys who sold ETH too soon after picking up some at $10$11. Till date, they're still regretting that move. My point is, If the market conditions are good and favourable (like a bull run kinda scenario) then you just probably hold for a bit longer. But don't get too greedy and cocky about it too. At the end of the day, no strategy works at 100% every time.