Spark Plug Life - when to replace?

[pcastine]

My apologies if I've missed this somewhere already posted (I've searched...)

What are typical service lives for aviation spark plugs? When are they typically replaced?

I have about 400 hours (+/-) on the plugs in my engine currently. Recently, I've had minor issues with 2 different cylinders, (engine running rough) that were traced back to bad plugs each time. Wondering if I should just go ahead and replace all of the plugs? Resistance checks ok on the others I've looked at (I haven't checked them all) and visually they look normal.

Thanks for any advice,
Pete

 

[n9617D]

 

[pcastine]

 

[Domenick]

 

[rwyerosk]

 

[JimW]

 

[Arrow4]

 

[flyguydon]

 

[philiplane]

 

[pcastine]