"I just replaced the one in my '96.
I wouldn't worry about it until funny things start happening"
A lot depends on who uses the car and for what. If it's a second car
that isn't a priority, isn't likely to result in a tow charge if it
goes dead in the wrong place, etc., is driven in moderate climates,
then maybe I'd wait. On the other hand, if it's a car that you use to
visit clients, need it to always start reliably, is driven in cold
winters by someone who will have to call for road service when it
fails, then I'd replace it now.