Well it’ll stop even less than that. Mass shooters plan for months, the law isn’t intended for that. It is meant to stop “crimes of passion” (read: killing your wife), but all that would happen is they prevent this time (or he goes all Chris Benoit), then he picks up his gun 10 days later, and next time he’s in a wife killin’ mood he’s all prepared.
In fact, statistically, according to the ATF, average “Time to crime” of a firearm (time from purchase to when it ends up involved at a crime scene) is 11 years. That’s a bit longer than 10 days.