Like I said, it's just an idea planned atm, the exact details haven't been considered, but it will probably be something like - the first time you download a modification after it's last updated time, a flag will be set - then x days later, if the flag is set, and you haven't marked it as installed, a PM will be sent, and the flag unset again. I don't think that will mean 'loads' of PM's flying about.