Dave
Lord Howe, being an isolated island in the middle of the ocean can be pretty windy at times, particularly in winter (AKA the 'windy season' to the locals). My guesstimation is that in summer you will probably get 4-5 out of 7 days outside the lagoon, while in winter it is more like 3 from 7. The good thing about Lord Howe is that there is heaps of land based fishing when the outside stuff isn't possible. I worked over there for a few months several years ago (before jigging and popping gear really existed). I did a lot of livebaiting and flyfishing off the rocks getting busted off and occasionally landing good kings and yellowfin. I would love to be able to throw stickbaits off the rocks at the massive kings and beer barrel yellowfin that swim past. As an example, the biggest King caught off the rocks there at the time was about 44kg, landed by an old local on a 200lb handline. Best yellowfin off the rocks was 50kg, caught by my mate Jason and myself on a 15kg stand up outfit. My PB king off the rocks was only about 16kg but I was blown away by bigger fish almost every time I went out.
As a time to go, I would pick from about christmas to April - the weather is the most stable and the greatest number of fishing options are available. From memory, the big kings are pretty much a year round thing, but there are plenty of other pelagics around while the warm currents are running in Summer and Autumn. Operators I would speak to would be Gary Crombie (Oblivienne sportfishing) and Keith (can't remember the surname) at Ocean View apartments.
Cheers
Duncan