The first MN reward
The average time it takes to receive the first reward is calculated in hours with the formula millions_locked_before_your_MN_is_enabled*2.6/60. At the time of writing the total number of millions that are locked in MNs is 2,500 million, so 2500*2.6/60=108.33 hours or approximately 4 days, 12 hours and 20 minutes.