30% profit is not bad if you invested a lot of money in altcoins. And what if the investment was about a hundred of dollars? is there any reason then to sell the cryptocurrency? Here, you should better reinvest.
Well, the question here is time. How long did it take him to get this 30% profit? Of course, if he makes 30% profit daily, reinvesting is the best he can do. But still there are some coins that should be just held, not sold after the first profit. It's a complex question and it couldn't be answered just by one value which is the % of profit.