I think you will be taxed when you sell your Bitcoins for other fiat. For example you bought 100 Bitcoin at $5 each and now they are worth a total of $560,000 then you will be taxed capital gain of your Bitcoins if you happen to sell all of your Bitcoin stash, otherwise you won't be taxed.
I don't think that you will be taxed even you will sell your bitcoin for the other type of currency because bitcoin is decentralized and taxing bitcoin is not covered in the law of the government so it is not that easy for bitcoin to get into taxing process because no one can dictate bitcoin because bitcoin is allergic to centralized people and it will still remain decentralized and even in the future, i can say that it will not get taxed.