Is it unreasonable to do a daily garbage collect as a cron job or similar on each machine?
If your threshold is 30 days, you could make it weekly, etc.
'ipfs repo gc'
Once unpinned I believe it should get garbage collected if you run it manually. After gc'd from both your storage nodes and the gateways, assuming no one outside your network is sharing it, it should be effectively gone.
I'm interested in a time based unpinning, but not time-based garbage collection. I'm probably simply going to keep a separate table of when pins were made.