How long will it be before our current Big Bang theory is out of date?

One thing in science is that you can go out and do an experiment about peoples' claims. If the facts don't fit the theory, then the theory goes straight out the window. Huxley once said that there's nothing more tragic in science than the slaying of a beautiful hypothesis by an ugly fact. That's how science works. Scientists are not so arrogant as to say they know everything, but there are things in cosmology that are absolutely certain, for instance the influence of radiation. There are other things that we don't know much about, however, for instance dark matter, and cosmologists are happy to admit this. At the moment it's impossible to say.