As quick as a flash.

In a heartbeat.

Usually written or spoken as: "In the blink of an eye" meaning extremely quickly.

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Synonyms are listed with the informal first and the formal at the end:

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PDQ (means Pretty Damn Quickly)
tout de suite (French but used in English)

<o:p></o:p>

<o:p> </o:p>

soon
in a flash
in a jiffy
in nothing flat
on the double
pronto
right now
at short notice
this instant
this minute
instantly
promptly
rapidly
straight away
straight off
without delay
without hesitation

immediately

instantaneously<o:p></o:p>

forthwith<o:p></o:p>

unhesitatingly

<o:p>
</o:p>

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<o:p>Plus the other excellent suggestions from Marc, Shannon and Elsa</o:p>

Faster than the speed of light!

"Faster than you can say....." This you can use with any word really, it's a common expression. "She got out of there faster than you can say breakfast!" (Meaning she didn't stay for breakfast, she left so quickly.)