Well the answer may surprise you. It’s a very short amount of time.
In this study they found the recommended threshold for retina injury via a LED curing light was breached in 62 seconds for a 325mW curing light and in 27 seconds for a 740 mW curing light
These times are based on the recommendations from the article below.
American Conference of Governmental Industrial Hygienists. Threshold limit values for chemical substances and physical agents and biological exposure indices. Cincinnati, OH: American Conference of Governmental Industrial Hygienists; 2008. p. 146-55.
If these recommendations are accurate then the time of exposure at 30cm away from light that you can look at light is certainly way less than 27 seconds. Although I am not sure I completely trust the recommendations of what is a safe level and what is not, if this is even close the light is much more dangerous than I previously believed.
Update CR just covered this topic and is also advising that the LED headlights which have a lot of blue range light may damage eyes too.