The real question is not how many years or how long will it take bitcoin to reach the number one spot in terms of currencies, the real question would be how bitcoin will achieve that, the reason why bitcoin hasn't reached the top yet is because it is still not legalized, governments and countries are not accepting it as a way for paying for things so people prefer not to use it, and major investors don't want to put money into it in fear of its instability, i think that if bitcoin gets accepted by all the big countries in the world it would defiantly reach the top and be used by everyone in the world for it is a currency full of advantages and benefits.