When is a good time to take profits on a rising asset or sell on a falling asset? How much should one hold as cash ready to invest?

Assuming you are in it for investing rather than trading. You would buy when it's undervalued, hold till it reaches it's potential or something fundamentally changes.

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For example, this pharma co. is developing a new drug, you like the idea and think it has potential, so you buy. Then came a delay in the release, then another delay and another delay. It's up to you if you think the delay is just a bump or bullsh*t. If it's a bump then you hold till the drug gets to market and make $, but if you conclude it's something more sinister, then it's time to go.

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The skill comes in the judgment and speed of your conclusions. No magic formula, unfortunately.

When price is less than intrinsic value, buy

when price is more than intrinsic value, sell