my strategy with altcoins are very simple. When my coin 2x in price, i just sell 50% i.e i take out my initial investment. In that way i ensure no loss. The according to market i can plan when i need to sell the rest.
I think many use a strategy like this because we are supposed to trade it will apply that method, but if you wait to get a profit of 2x the price when you buy it will be very long, considering that not all altcoins have such good price movements, maybe an increase of 1 , 5 is enough to partially sell it.