I forget what the plugs are, probably some NOS Bosch that I got cheap from a local garage a few years ago.
Otherwise they would be NGK's
I think that in my stock 1200 engine they will last untill the sun grows cold.

Twin Delorto's? I suspect that you wouldn't have enough manifold vacuum to suck a fizzy drink up a straw anyway & since these carbs do not have provision for ported vacuum, it becomes a somewhat moot point.