How long will hydrogen last?

Answer 1

We don't know.

Hydrogen is a proton and an electron. Electrons are fundamental and, as far as we know, cannot decay. Protons are not fundamental and some theories predict that they could decay, but experiments looking for proton decay have determined that, if it does happen, it cannot happen with a half-life of less than 10^34 years. That is ten thousand million million million million million years. If hydrogen goes away via proton decay, it is going to be around for a VERY long time and possibly forever.

If it's the Big Rip ... well, again, we don't know. There's a certain parameter in an equation which is very close to -1, the timing of the Big Rip (and hydrogen will be one of the very last things to go, just before protons themselves) depends on exactly how close it is to one. The closer it is to one, the longer before the Big Rip; if it's exactly one (which is a possibility) the Big Rip will never occur. A sample calculation with it set at about -1.5 ... that is, not at all close to -1 ... gives at least 22 billion years before there's no more hydrogen.

In either case, it's long enough that it's not something that you will ever need to worry about personally.