Most miles driven before changing spark plugs?

I'm at 137K and it still runs perfect. I see on the attached it states to change them at 120K. What kind of problems occur if they're not changed.

Note: I do the fluid changes religiously but not sure if plugs need to be changed as it runs like the day I bought it new in 2019.

 

Change them, and while you’re getting intimate with your truck, clean the maf and throttle body with proper cleaning sprays. A little maintenance goes a long way

 

I recently changed mine at 120k also did the pvc. I got 1/10 now 14.6 mpg improvement so I don’t think they were bad, but I’m giving this truck to my son in 4 years so I did the maintenance. I’m sure your fine.

 

205k miles.
I'm pretty sure this thing is still on factory radiator hoses and all that. When I get the check engine for a misfire is when I'll worry about plugs and coils and hopefully they won't be seized in the block by then.