What would be telltale signs that quantum computers become imminent and sizable danger to classical cryptography in commercial applications ?
Make classical cryptography consist of symmetric algorithms (block ciphers, hashes), and asymmetric algorithms assuming hardness of factorization or discrete logarithm in $\Bbb Z_p$ or Elliptic Curve, with parameters large enough to reach security against classical algorithms run on world-class classical computers. Perhaps, assume access to all results in the field of quantum computing (even secret ones: you are CIA analyst with eyes everywhere).
I suggest "imminent" to be 2 years and "sizable" to be something worth betting for if winnings are 100 times the bet; but feel free to parametrize or change that.
Please develop rational arguments (preferably quantitative) e.g. based on categorizing what's achievable using various breeds of quantum computers, what inherently limits progress in the field, and comparison to other technical advances.
Motivation: this May 18, 2018 article quotes the director of a research lab involved in the development of quantum computers:
"Anyone that wants to make sure that their data is protected for longer than 10 years should move to alternate forms of encryption now"
and based on that source, the journalist writes:
Quantum computers will be able to instantly break the encryption of sensitive data protected by today's strongest security (..) This could happen in a little more than five years because of advances in quantum computer technologies.