I can't tell you when to buy or sell, but when people ask me this I ask them are they investing or day trading.

If it's the former, you hold until it reaches some longer term profit target. You won't be concerned with day to day fluctuations in price because you think long term it will be much higher than you bought in.

If it's the latter, then set a short term profit target and a stop loss. That way you manage your risk and take a small loss if you are wrong, and you make a modest amount of profit in a short period if you are right.