To answer your specific question, one answer is the workplace exposure limit, above which you must not allow anyone to be exposed (while at work). This value for formaldehyde is 2 parts per million. That said, this also takes into account dilution in room atmospheres, etc., not drawing it directly into your lungs, so i suspect the harmful level in the respiratory tract is considerably lower than 2ppm.
Further to this, the article cited by
@Brighton Sky Blue includes the sentence "We show that, within the tested e-cigarette brands, thermal decomposition of flavoring compounds dominates formation of aldehydes during vaping,
producing levels that exceed occupational safety standards."
If your employer would be prosecuted for exposing you to such levels, why on earth would you choose to inhale it directly into your lungs?