Understanding the Salesforce Campaign Influence Time Frame

Understanding the First Associated Date : 

To understand how the Time Frame is calculated, you first need to understand what the First Associated date is and how it's calculated. 

The First Associated date is the date that a Contact is added to a Campaign. In other words, the First Associated date is the same as the Campaign Member created date. 

That being said, the criteria for which Campaigns will be associated to which Opportunities is as follows

(Opportunity Created Date - Campaign Member Created Date (First Associated Date)) should be less than the Time Frame. 

Scenario 1
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Opportunity created date: September 15, 2014
Campaign member created date (First Associated date): December 10 2014
Time Frame: 10 days

Opportunity Created Date (September 15 2014) - Campaign Member Created Date (December 10 2014) =  -86

-86 is less than 10 (Time Frame) so the Campaign is considered influential to the Opportunity. 

Scenario 2
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Campaign Member created date: March 19 2014
Opportunity Created Date: August 12 2014
Time Frame: 10 days

Opportunity Created Date (August 12 2014) - Campaign Member Created Date (March 19 2014) =  146

146 is greater than 10 so this Campaign is not considered influential

The scenarios above assume the Opportunity has the Contact Role which is the same Contact that acts as the Campaign Member.