First factor that makes me decide to sell is when i badly need the money and no other source of funds for that urgent needs, 2nd is when the price reaches high to the moon and you are already satisfied of the increase of the money you invested, thats the time i may decide to sell and wait again for right time of buying for another holding...
You have got very nice plans with bitcoin and I am sure that you will succeed in doing so. Bitcoin is going to increase in value by next year and 25,000 dollars is easy for bitcoin to achieve by the mid of coming year, 2018. I am also in favor of collecting bitcoin as much as you can regardless of the prices it is at because the fact is that you will earn big profit with it after holding it for some time. It is never going to put you in loss.