Would someone help me with the following expression ?
"I will be ready before you can say ( )."
Now, "Jack Robinson" used to be in the parenthesis, but it appears that anything can be put in there
if the word(s) is(are) related to the situation you are in. Could someone please give me some examples?
And if you still use "Jack Robinson", would you tell me how old you are if you do not mind？

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This is a term often used in comedy to assert some form of irony. with the meaning of "I'll get it done so quick it'll amaze you"

We use it every now and then in English when talking to friends but only really in a joking sense

For Example..
Me: Hey, Could you go and pick up the Pizza's from the shop?
Friend: Sure I'll have it done before you can say 'Pepperoni'

Me: I've got something wrong with my computer, could you check it out for me?
Friend: Sure I'll have it done before you can say "Norton Anti-virus"
*Note Norton Anti-Virus is a notoriously well known Anti-Virus program (for the wrong reasons)

Hope that helps a bit!