Since this is included in trunk under a different name, this can be tracked with the 0.11 milestone.
title[mod_mam_muc] add Archive expiry to mod_muc_mam: Archive expiry
We're working towards the release of trunk, and it is unlikely this feature will get in within that time, unless someone contributes it.
When is a release of 0.11 expected? Until when would you need this patch?
There's no date set yet, but hopefully before the end of August.
The patch would be considered until a week or two before the release, but inclusion would depend on how invasive the patch is (more code/changes == more likely to introduce bugs).
Backporting the better cleanup mechanism from trunk into 0.11 for mod_mam and mod_muc_mam