How much is the IEEE standard for Software Requirement Specification
used in real world?

I work as a senior developer, and I used it two weeks ago.

It's a very good standard, and very usable in the real world.

However, it's not used very often, mainly (IMO) because of a sort of
snobbery that says "My software is special, and outside standards
won't be useful to it."  The reality is that the IEEE document
contains advice on how to customize it to your particular
circumstances.

As an anecdotal data point, in my 27 years of experience, mostly in
small companies in Silicon Valley, I have never seen it used at all. 
From the amount I (don't) hear about it, I would agree with
nickargall-ga that it is rarely used.