Well ~4 years.  The blockchain measures everything in blocks.  We have 210,000 blocks between subsidy cuts.  We have 2016 blocks between retargets.   Now why Satoshi didn't make the reward reduction period a multiple of the retarget period?  I will never know.

If the target of an average of 10 minutes per block is maintained that is ~1458 days (2 days short of 4 years).