If you are referring to bitcoin as becoming the cash for a daily basis that will be accepted anywhere in the world in a p2p format, it could take ages because maybe the technology will not be there to have low amount settlements become practical on a global scale.

It could function as a global settlement currency between big players though. It is impossible to do with gold, but it would perfectly be possible to have things like interbank settlements with bitcoin. But it is a similarly long time horizon I think as for settlements between big players, bitcoin would have to have a very low volatility and it is quite a way from here to get there. But the higher the market capitalization and volume or liquidity, and the lower the volatility, the more likely it is that bitcoin could become one center of attention for settlements of various sorts.