I know this is an old post, but I created the below search query below. Should work for anyone looking for their average over the past 7 days. It's a big ugly search and I'm sure there is a way to simply this or to heavily utilize eval but this is what I ended up with. Hope it helps everyone.
index=_internal source=*license_usage.log type=usage
| eval GB = b/1024/1024/1024
| timechart span=1d sum(GB) by pool
| eval Time=strftime(_time, "%m/%d/%Y")
| fieldformat auto_generated_pool_enterprise=tostring(auto_generated_pool_enterprise, "commas"). " GB"
| fields - _time
| addcoltotals auto_generated_pool_enterprise label=Total labelfield=weeklytotal
| fields Time,auto_generated_pool_enterprise, weeklytotal
| search weeklytotal=Total auto_generated_pool_enterprise>1*GB
| eval weekly_average=(auto_generated_pool_enterprise/7)
|eval Total_weekly_average=round(weekly_average,2)
| table Total_weekly_average
| rename Total_weekly_average AS "Average over the past 7 days"
... View more