The lifespan of bitcoin isn't really explainable and with lots of speculations here and there. The understanding that it is a technology that has come to stay should bring about its adoption/acceptance and adaptation/usage. It is a digital currency thriving on the block chain technology. So, a better question is how will block chain stand the test of time. Well, a lot of people and some government are now looking into studying it and how it can be relevant/helpful to their mode of operation; in getting the best output.