I've read in some places that the next halving event will be on 2016. How can this be estimated?
The Bitcoin reward schedule follows a predetermined pattern, see Controlled supply from the Bitcoin wiki.
The next reward drop will happen at block number 420000. The current block number at the time of writing this answer is 318662. There are about 101338 blocks remaining. At approximately 10 minutes per block, there are approximately 1008 blocks per week, and 101338 divided by 1008 is about 100.5. So you can expect the next reward drop approximately 100 weeks from now, or around August 2016.
The http://bitcoinclock.com site does this calculation for you and provides an estimate right at the top. At the time of writing this answer, it says:
Reward-Drop ETA: 2016-08-05 12:47:44 UTC (100 weeks, 3 days, 17 hours, 40 minutes)
On average, since the creation of Bitcoin, a new block has been created every 9 minutes and 20 seconds. This is 7% faster than the correct time of 10 minutes. Taking that into account, I predict that the next one will be at Jun 20, 2016.
Reward-Drop ETA date: 2016-07-09 16:44
Date extracted from: https://bitcoin.halving.info/
The halving date is dynamic, each 210.000 mined blocks a halving event occurs.
(Blocks for halving - Actual total mined block ) * Time for block generation
The next halving event occurs when the Bitcoin network reaches a total amount of 420,000 mined blocks.
The aproximate time between blocks is 9.47 minutes
(420,000 - x) * 9.47 = Time until next halving event in minutes
Chiming in to confirm that halving is approaching faster than scheduled - it has been 30 days since @Greg_Hewgill posted the estimate from Bitcoinclock. Currently it displays:
Reward-Drop ETA: 2016-08-02 15:26:20 UTC (96 weeks, 8 hours, 40 minutes)
In the last 30 days, Bitcoinclock has reduced its estimated date until next halving by 3 days / 10%.