Spark plugs, except the cases where some material failure occurs will actually take an eternity to fail and not being able to fire at all. At least in normal engines with normal operation.
What actually happens is they wear out, or get cluttered with deposits.
The first case of wear occurs when their terminal metallic points (a small cylinder-needle in the middle and a small flap on the side) melt or bent slowly over time as a result of constantly been under heat, pressure and current flow. This causes the gap between them to change, changing as well the "quality" of the spark produced and leading to lower quality ignition on each cycle. This wear occurs VERY slowly (especially in Iridium type spark plugs) and it might actually be acceptable even after 100K km in some cases.
The second case occurs, when on a not perfectly adjusted motor (and NONE is...) some deposits (usually carbon based) form on these endpoints either by small amounts of oil in the piston chamber, by "dirty" gasoline, by incomplete burn or just by normal exhaust fumes trapped in the chamber.
Long story short. Spark plugs get worn out by time AND usage and at some point they begin to work less effective (don't underestimate this state's consumption cost). If you want to save some money you can always take them out occasionally and clean them with a brush and measure - recalibrate their gap.
Still to save yourself from this whole fuzz, just get some new spark plugs installed as manufacturer suggests. You don't want a 30$ change cost to make your vehicle not operational in any (even non-destructive) way.
HERE: you will find a nice guide on how to "read" your spark plugs condition and determine your possible engine issues... https://www.crankshaftcoalition.com/wiki/Reading_spark_plugs