Like ADA for example, I have noticed that when it peaks, it trundles on top for a while then will drop to usually a higher low, which is good, but I missed an opportunity to buy back more coins or simply take advantage of a pump.
If the hypothetical coin that you predict about is ADA then I would cash out at the chance. ADA and other coins that were pumped by a notorious youtuber called Suppoman (Supposcammer, if you please) are a part of a pump and dump group due to the following of this youtuber. He was previously banned and is now back promoting some paid coins.
In general if you ever see >20% profits on any altcoin, sell it. For bitcoin a 20% gain is a huge amount because the base price of bitcoin is much higher than any altcoin compared, so you wont see 20% gains in bitcoin or close very soon, but a 5% gain there is quite equivalent. In case you have faith in the coin to go even up, you are free to hold the coin. But trust me, holding altcoins have been much less profitable than holding bitcoin.