How it works...
Every new block the blockchain contains a bit less coins.
How long until rewards hit zero?
After 2 years, 1,050,000 blocks, the rewards will stay fixed at 250 TNA
How accurate are those timelines
This is all dependent on the number of blocks, not calendar time. If the block speed is faster than one minute, it will be sooner. If slower, it will be longer than 2 years.