There are different strains and each person has a different immune system, but often I hear phrases like "Oh well, we already kissed, so I'm probably already sick". How faulty (if at all) is this belief? Is there a general guideline on how much of an interaction you can have with an infected person before you are probably sick?
No, it can't be assumed. You should continue with prophylaxis methods to prevent the spread.
I have found two secondary sources that point to primary sources that, alas, I don't have access to. This puts me in the dangerous position of summarising someone else's summary. Sorry.
First up we have Donald A Goldmann, Transmission of viral respiratory infections in the home, Pediatric Infectious Disease Journal: October 2000 - Volume 19 - Issue 10 - pp S97-S102.
Interestingly subsequent work demonstrated that it was exceedingly difficult to transmit the virus orally or by kissing.
He attributes that work to:
Hendley JO, Wenzel RP, Gwaltney JM Jr. Transmission of rhinovirus colds by self-inoculation. N Engl J Med 1973; 288: 1361-4.
I have been unable to confirm that this paper says this. Has anyone access?
This patent for a Door Handle Cover cites J. Owen Hendley and Jack M. Gwaltney, Jr., Mechanisms of transmission of rhinovirus infections. Epidemiologic Reviews, Vol 10, 1988
The patent authors claim that the paper authors claim:
Colds are not caught by kissing as cold viruses do not infect the mouth and saliva contains very little virus. When volunteers infected with common cold virus kissed 'cold-free' volunteers for up to 1.5 minutes, only one case of cross infection occurred in 16 trials.
So, good news! You can relax, you probably haven't obtained the cold after kissing someone. (Or should this be "Don't relax! Stay vigilant."?)