Sliding Window Average For Multiple Time Periods - SQL Server

I have a SQL query that returns the average time to signup for o开发者_开发问答ur users who signed up in the last 30 days:

select avg(datediff(dd, acquisitiontime,createdate))  from users where 
createdate > getdate() - 30 and
acquisitionmedium = 'cpc' and
acquisitionsource = 'google' and
acquisitiontime is not null

I want to watch how this has changed over time.

How do I change this query so that I can spit out a table with (Month, Avg Time to Signup for that Month)?

select
    DATEADD(month, -n.number, getdate()) OneMonthFromThisDate,
    avg(datediff(dd, acquisitiontime, createdate)) AverageInThisMonth
from users
join master..spt_values n on n.type='P' and n.number between 1 and 24
where createdate >  DATEADD(month, -n.number, getdate())
  and createdate <= DATEADD(month, 1-n.number, getdate())
  and acquisitionmedium = 'cpc'
  and acquisitionsource = 'google'
  and acquisitiontime is not null
group by n.number, DATEADD(month, -n.number, getdate())
order by n.number

This puts the most recent first.