See , Bitcoin's price cannot be stable for even a day and then you are talking about 5 years , that's pointless.
Here by stable I mean the exact same value like ( 5000-5100)$ .
We just saw the price go up in a couple of days and now you are coming to the conclusion that this is the final price.
I think by the time goes the price price will fluctuate up and down ,but the increase will be dominant over the decrease.

There is right time to buy or sell Bitcoin. I mean that right time cannot be guaranteed by someone.
But I would say that buy the coins when price is lowest and sell when the price reaches its peak value , as for now the price is above 5000$ which is the right time to sell the Bitcoin if you are interested.