Sigma published a good techincal doc about rockers. Worth a read.
In summary - they built a motor with a special cam in it and it happened that the oilway plug was not fitted to that particular cam and they didn;t realise it. 90 seconds after starting the engine the oil started coming out of the end of the cam. Hence the estimate of 90 seconds from cold to get the oil up to the rear cylinder.
http://www.sigmaperformance.com/rockers.html