I carried mine for 3 1/2 years. The car stalled once, and restarted on the third try.
I went home, changed it, and all has been well ever since.

If you have a spare, you aren't going to be stranded as much as delayed. You could change it and carry the old one as a spare if you are shaky about changing it in a parking lot or by the road side someday. But usually, they get intermittent, not often does anyone say theirs failed completely the first time it acted up.