27.7 days is very long time, so you better hope majority people won't move their vulnerable UTXO at similar time.
yeah and i figure normal day to day TXs on top of that.. so all those need to be squeezed in too.
It makes sense to consider daily/regular TX. But when mempool is congested (people must pay far higher fee rate to get their TX confirmed within reasonable time), some people would make less TX or even decide not to make TX. It add more variable, which makes the calculation become more difficult.
For reference,
https://statoshi.info/d/000000002/blocks?orgId=1&from=now-3M&to=now&timezone=browser shows blocks in last 3 months is about 91.4% filled. It's based on 3658727 average block weight size.
but can wallets and such be made so that, say, a TX from a quantum vulnerable addy to any other addy, quantum safe or not gets change back in a quantum safe addy? at least some day to day TXs would get rolled in for free that way, kinda.
I agree with @Satofan44. They (the one who create TX) must use wallet that can generate QC-resistant address and use it as change address automatically.