The best I can tell you is that dry firing with out a snap cap will wear out the firing pin spring faster. Now, how many dry fires does it take to do this? 1000, 10,000, 100,000? IMHO occasional dry firing doesn't hurt.
It is my understanding that there is no need to worry about dry firing modern made pistols. I have extensively dry fired all my pistols and never had a problem. This includes revolvers, 1911s, P99s and my PPK!
I have never even heard of anyone having a problem dry firing.