The best guide to servicing is the manufacturer's recommendation.
If Alfa specifies 30,000 miles (50,000 km) then that's the change interval I would use. It seems very short for iridium plugs although there are two designs of iridium plug and only one of those two has the very long service life.
The picture of the iridium plug is of the shorter service life version. The long life version has a needle sized centre electrode and a similar "slice" of the same diameter material welded under the ground electrode forming the gap.
NGK markets the former type as Iridium IX and the latter as Iridium Laser. NGK welds a disc of platinum under the ground in the Laser version. I've seen long life Denso plugs using a tiny piece of iridium "wire" identical to the centre electrode welded to the underside of the ground which I understand extends the service life of those plugs to 100,000 miles.
The two designs are interchangeable but the long life plug is much more expensive.
For my supercharged Jaguar which develops 340 bhp from its 3.0 litre engine the change interval is 160,000 km (100,000 miles) with no time interval. Just checked NGK parts listing and sure enough they specify ONLY the laser iridium plug: change interval 100,000 miles. Interestingly, the specified plug is also "indexed" which formerly was only racing engine specification. That means the orientation of the gap is critical. The manufacture requires that when the plug is threaded fully in and torqued EXACTLY to spec the gap faces a particular direction. None of this: "screw in hand tight and then an additional quarter turn" estimated installation torque! You need an in /lb torque wrench.
Plugs are interesting devices. In the old days we changed the copper core plugs in our turbo engines every second oil change: at 2x7,500 km ( yup, conventional oil and the filter were all used up in 4,000 miles and the plugs gone in 8,000 miles). Mind you, ten years earlier that was the tune up and oil change interval for a pedestrian OHV engine!
Don't remind me about points and condensers.