Car Loans 15-45 day rule

Hello.

I want to start shopping around for car loans. Can someone explain in more detail the 15-45 day rule when the effects on your credit score may be lumped into one?  Also why is it such a range?  Should I play it safe and apply for a loan only 2 weeks before I plan to buy a car? Or do I truly have 45 days? 

Thanks.