the NAT ops bulletin checklist says 30 min, is there a different ruling on this one?
30 mins is the correct figure.
Then why does Iceland Radio ask "Whats your next OCA" if you dont tell him on initial contact. May be Reykjavik Control does not tell Iceland about the aircraft's next OCA and Iceland radio believes that they have to pass the complete info to Reykjavik, so they need to know the next OCA.
The reason you are asked your next OCA is that as you are CPDLC/ADS there is no further reason to 'talk' to the radio station prior to exiting that OCA and entering the next. You should be given the contact HF for the next OCA when you make the initial SELCAL with for example:
"Shanwick BIGJET123, CPDLC, Gander next"
"Roger BIGJET123, at 30W contact Gander on...."