In Approximately 4 years at block 2,100,000 the halving will take place and the reward per block will drop from 5,000 to 2,500 Raven. This will continue with each subsequent 2,100,000 blocks until the reward amount can no longer be halved.
How it works...
Here is more details on how that actually works.
2,100,000 blocks = 1458.33333 days @ 1 minute blocks
1458.33333 / 365.25 = 3.992699 years (with one leap year)
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 4 years.
Example: By counting blocks, Raven is currently at 108.39 days, but in calendar time, we are currently at 92 days. This difference was caused by our mining power being increased almost every difficulty adjustment.