I have mentioned this in other threads Crypto is like venture capital, very volatile snd some of the coin can be expected to produce geometric returns and others can be expected to fall to zero. In between there will be massive volatility.

Given the high volatility and possibility of unforeseen events like exchange hacks or exchange bankruptcies, it feel its important to diversify across the number of coins (>30 if possible), number of exchanges, number of wallets (some at exchanges, some hardware wallets) and allocation to different consensus methodologies (e.g. not invest all the capital in PoW coins). This is particularly important if the investor has a large portfolio either in absolute dollar terms or as a percentage of his or her net worth.    

In terms of when to exit, i think the time frame should be dictated by required spending. If you need the capital to purchase a house or pay for retirement expenses then so be it. But other words I feel buy and hold is a good strategy and this will tend to be tax efficient.