Depends on the market situation and what type of your trading. For example, I am a daily trader. If the market is in the uptrend, I will sell my altcoins if the price can increase 50% - 80%. But if the market is in a downtrend like in the current days, I mostly take profits once the price can increase 20% - 30%. I got fewer profits because most altcoin prices tend to decrease and hard to increase fast. However, it is my own strategy, other traders may be different.