If anything, decay should be upkeep based, needing the materials of the structure.
Heres my idea:

Give T1, T2 and T3 structures a maximum decay time for their tier, 48 hours per tier for example. From here, the highest decay can be refreshed with your presence is 24 hours.
To boost up to the maximum decay capacity of your tier, you’ve got to build a new bench that can only snap onto a foundation or ceiling piece. This bench has a single slot, and will gradually consume tier appropriate resources to add time onto your structure, up to the cap.

And thats the base.
We could add additional rules to it, such as more resources required per x amount of blocks in that particular structure, no timer decay while the owner/clan member is online, perhaps even an increasing material cost per cumulative hour the structure has been decaying for previously, so even vaults worth of resources will rapidly be chewed up if people only log for a few minutes to “feed” the bench over a month or so.