The perennial traders' question. Can't calculate it as x% in 300 days as that ignores the losses we also take. Samson is partly right - depends on the risk you took. More importantly, depends on your analysis of the next move. If it may go down (>50%), sell. If it looks >50% it will go up, hold, and move your stop up.
that's an example of a total bullshit line. i've met tons of traders who lost money taking profits too soon, out of line with risk, etc etc. the world does not need more trading platitudes.....