I've never had to replace plugs, But the reason I changed them out was recently the car was jerky from cold and was missing sometimes around 3k in third and fourth. The old plugs had 45k and the gap was 0.9mm. The car definitely feels smoother.
Brilliant Ngk iridium
The item was well priced but what did it for me Mountune pre-gap the spark plugs for you. So its simply boot them straight in and off you go