How often is it used ?
How often is the expression " Bob's your uncle "used in the UK?
It is not formal is it?

Question

Answer

It's old and fairly common, so much so that it has spawned take-offs, such as "Robert is your auntie's brother".  Yes, it is informal.  My British information is dated, but it's definitely still used in Canada.

For more information on it:  
https://en.wikipedia.org/wiki/Bob%27s_your_uncle
http://www.phrases.org.uk/meanings/bobs-your-uncle.html

Answer

I do not know about the UK but in the USA is is not very common, it is understood by any english speaker, but it is nether obscure or common you hear it every now and then

Answer

Mistake :s  I wanted to post it in the English section.