what is that exactly? Isnt the total paper money used for this contest or the current value of a players portfolio?

Ultimately, there is no definite or right time to buy or sell any security. It really depends on your risk tolerance and how much you are willing to gain or lose on a trade. That is why it is best to set boundaries or parameters for when you trade. Once a stock looses/gains a certain percentage, you will know to sell or buy at that point. lets say your confort level for a certain stock is 2%. You can set alerts to let you know when the stock has gained or lost 2% and then decide to buy or sell. The same thing applies to the price. its best to watch a stock or study its patterns to know how low it usually goes, or hits the floor (support), or how high (hits resitance).