Calculating Avg Of Days Broken Down By Period

Aim – Calculate the number of days between CreatedDate and [Date_Docs_In_Complete__c],count how many Ids, Fall within a specific month by year, and then work out the avg number of days it took for each month

My query so far

select
ID,
left(CreatedDate,10) as CreatedDate,
left([Date_Docs_In_Complete__c],10) as [Date_Docs_In_Complete__c],
DATEDIFF(day,CreatedDate,[Date_Docs_In_Complete__c]) as Days
from #build

Produces following results

IDCreatedDateDate_Docs_In_Complete__cDays
0063000000ausKGAAY2010-03-262013-07-161208
0063000000mC359AAC2011-06-302013-07-03734
0063000000oyaSPAAY2011-11-292013-07-18597

Desired outcome results would be

Year Month Date_Docs_In_Complete__c Total Days Avg_Days
2013 07 3 2539846.3