http://korma.wmflabs.org/browser/scr.html has "Age of open changesets (monthly snapshots)" per month (also under "KPI: Overview").
In T132421 we decided that we'd like to have something similar on wikimedia.biterg.io.
Note that Gerrit-Backlog currently offers "Backlog: NEW Changesets waiting to be Closed" (a bar means "number of patchsets created in that very week/month which are still open"); and "Backlog: Accumulated Days waiting to be Closed" (I don't fully understand what the latter expresses).
