I was farming these a few months back, and killing at 45-50 mobs per hour, it was taking 8-10 days per drop. That comes out to be 1 drop every 12,000 mobs or so over a span of 90 days. But on stretches it went as long as 25,000 kills before I saw a drop, sometimes as low as 1000 kills, but average came out to be about 12,000 for me. The quoted drop rate from a developer is 1/5000.
Rarely did I get a drop every 5000 mobs, even +/- 5000 mob deviation from 5000 it was tough to get a drop. What I did find is the special event ornaments throw of the drop rate off considerably, like the Erollisi/Innoruk ornaments that are dropping in game right now. Those are on the same loot table, so are the Stone of Tranquility (drop rate every 300 mobs on those), as well as Scale Ore. So anything they add onto that table is going to throw off drop rate, is what I think is happening.
I have no clue what the drop rate is on the TDS ones, but that is the CotF drop rate.