According to the stats:
Dogecoin is minted at their 91.45% (91,448,038,777.00 coins)
8.55% still remaining (8,551,961,223.00 coins aprox)

If the target is maintaining approximately 100,000,000,000 DOGE in circulation after 1.5 years (stats from the OP):
100,000,000,000 - 91,448,038,777.00 = 8,551,961,223 coins remaining

If 100,000,000,000 = 1'5 years
8,551,961,223 = X

X = (8,551,961,223 * 1'5) / 100,000,000,000 = 0,128279418345 years.

If 1 year = 365 days
0,128279418345 years = 46 days

But... this is not an exactly calculation!
Expected circulation it's not exact and can vary. That's what you're going to see it on that table according to the Wikipedia block schedule:


So... according to devs schedule published on the OP, 26 January 2015 (estimated) can be the date that you're looking for