Since when Audit Logging gets enabled

Hi,

I realized that audit report settings were blank for my site and i just enabled some of the settings to get audit reports. I guess I will have to wait for some days for that to take effect. 

I have two questions:

1. Why do we have to wait for some days for auditing to take effect ? and how many days we need to wait ?

2. Suppose I enable some auditing report ( say "Editing Items" ) today, in "Site Collection Audit Settings", once the audit report gets generated, will i get audit data for the past days or only from the day the "Editing Items" is enabled ?

Thank you.