how often do I need to tuneup or replace spark plugs? on 2005 Cadillac DeVille

Have not change my plugs in over 80K miles. Running fine. Just wondering

2 answers 2 comments
they claim 100k miles but i think its about time for yours as preventitive maint
Thank you.
on your car...100000 mi
Thank you.
How much is the usual charge, if go to Cadillac dealer?
Want some idea of the charges.