Currently Masternodes are rewarded approximately 7% p.a..
Masternodes get a share of the block-rewards, which currently are set to decline by 7.14 % a year. This typically happens in February/March each year.

Overall, this will approximately lead to Masternode rewards of:

03/22/18: 7.00% p.a.
04/09/19: 6.50% p.a.
04/27/20: 6.03% p.a.
05/15/21: 5.60% p.a.
06/02/22: 5.20% p.a.
06/20/23: 4.83% p.a.

The above has been derived from the following spreadsheet provided by Dash.org.