Hi Windward
I've got to go with the majority on this one and say about 8hrs minimum isn't far off.
But I will add that the majority of novice spitroasters have a tendency to get the roast too close to the heatsource,which means that the crackling(the best bit) either gets charred black, or you have to start carving thin slices of the meat off throughout the cooking,as no doubt you will be well aware that Pork is one of those meats that cannot be served pink.
When I do one which is getting rarer these days I'm not far off Bernies timings and start very early with the preparation and roasting.
I would suggest for a first go to go with your second choice of a lamb, as it contains more fat throughout the meat as opposed to pork,which has the majority of it's fat in the outer areas,and can dry out if you are forced to slice throughout the roasting. This will in turn mean you need to spend more attention on the basting,and lamb can be served pink.
But definately go for it as it's real fun and much more visually impressive than joints on a barbie or a pit roast.