I did a test very recently, actually (as I have considered heavy boxes as a possible way to do a player-run "quest" involving some chests that won't decay for a long time somewhere on the ground anywhere in the world).
What I found was that all items have a minimum decay time of 30 minutes, and beyond that, it scales at 1 minute per stone. HOWEVER, for some reason the heaviest items I specifically tested in this regard were 30, 50, and 54 stones, and they ALL decayed simultaneously at over ~ 100 minutes (if I remember correctly). I've also had containers with ~500 stones in them (had them locked down in order to get over 400 stones) last all night long. I imagine beyond 30 stones, it might round up to the next hour or something.
So, I guess the verdict is that beyond 30 stones, the timers may work differently than the 1 minute per stone deal. Would be an interesting thing to test for very heavy chests (100, 200, 400, 600, 1000 stones) if you are interested--it's real easy, just throw something on the ground where nobody will find it, hide your guy, and start a packet video. You have a timer built in to the packet video
.