When should I change the spark plugs?

:
mazda
2013 Sport Auto
I bought it with 88k.
Now its 110k.
When is the optimal time to change the sparks?
Can u tell if the previous owner changed the sparks?
Anyone know the OEM part #? Brand?
 
My handbook says the Mazda part numbers are:
LFJD-18-110 or L3Y2-18-110

I bought NGK plugs from rock auto when I did mine.
Called NGK - they told me Laser Iridium - LTR5BI-13. Stock# 90083
And to be changed every 75k

The plastic dust cover - do you just pull it off? I never opened it - tips?
 
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