Once the masternode gets the first block, the next ones come in turn according to the number of masternodes.
So it's a consistent 1.4 day rotation through the 2070 MNs for 1 block reward?
It could be less, for example, if some other masternodes are shut down or have a misconfigured firewall perhaps.
I'm not sure about the consensus algorithm for how the winning masternodes are chosen. I have had a masternode get no payments for a bit longer period and then have rewards come in more quickly. But it should average out.
At the moment I count 380 masternodes that haven't received payments. I may try to collect some more data and see how the situation changes.
EDIT: For me, masternode payments have started coming in after 90 hours.