Check out this how-to recipe for a rock solid use of User Defined Rollups in the #NPSP: calculating Consecutive Years Giving for a Contact. Take 20 - 30 minutes to add this to your org by following these step--by-step instructions  - then watch this space for details on how to use this to calculate average donor lifespan for your nonprofit directly in Salesforce.

And a MASSIVE hat tip to the wonderful @Beth Breisnes for her formula we used!

@Fundraising @Nonprofit and Education MindShare #Fundraising #User Defined Rollup 

Calculate Consecutive Years Giving using User Defined Rollups in the Salesforce.org Nonprofit Success Pack