There are studies on the effects of eugenol, but I don't know enough basic chemistry terminology to be able to make any guesses about the [amount of / percentage of in a solution] that it takes for cells to start dying as a result of exposure to it, but the stuff that I read seemed to imply that duration makes a difference - like 12 hours of exposure before toxic effects are likely to occur. What I don't know about is things like the amount of residue left in a person's lungs from vaping (similarly to tar, have to assume that there is some sort of residue) and the length of time that it remains.
I like to hope that not vaping cloves all the time minimizes the risks, but as n2xe alludes to, it is said that the toxicity of cloves is radically higher when inhaled versus being eaten. Considering the difference between the machine that is your stomach (lead-lined bowl) and the machine that is your lungs (plastic bags), it kind of makes sense.
I was really bummed to learn this stuff, because I've always loved the smell of cloves but was too paranoid to ever try them as a kid, and had kind of forgotten that they existed at all until I ran into the e-liquid version.
I am in no way trying to be an alarmist, I wouldn't want to scare anyone away from something that might not actually be any worse than any other flavors, but we're all lab rats with our e-cigs, so we should definitely be comparing notes and compiling information so that we can make the best choices and advise new converts in an informed way.
I'm no chemistry or anatomy expert, but I'm hoping that I've said exactly enough here for someone else to either come through later and correct me or fill in the blanks in what I've said above - I believe that what I am saying is accurate, but it isn't at all precise: the eugenol in our e-liquid may be too diluted to have the same effects that are mentioned in the studies that have been done, and I don't have the chemistry or math knowledge to be willing to make any guesses.
That said, I'm about to vape some Djarem Black, LOL.