That is difficult to exactly estimate because of luck. Every 210000 blocks the reward drops in half and the time between blocks *should* be around 10 minutes on average, but will never be exactly 10 minutes on average.

To calculate the next adjustment you could take 2100000 minutes - current block*10 - block of the last block reward reduction*10 (in our case 0, the genesis block)

The resulting number would be the number of minutes it should take under ideal conditions. To arrive at a better estimate you would have to do some cumbersome heuristics on the lag behind difficulty and actual hashrate which would only work if you know a better way to estimate it than the diffuclty adjustment algorithm does in the first place.

My uneducated guess for the 25 block reward reduction is the end of the maya calender