My 1985 Toyota Celica ST had ~100K when I bought her.
I changed all the hoses, plug wires, filters, fluids becaue
I got the car for $1800 and I wanted it to last.
I drover her 11 years, put over an aditional 270K before
the oddemter died in the 7th year and the plug wires are still
in great shape. The engine/tranny are great. The body is
very sad and while yoy won't get wet riding in her, it was time
for a "New" car.
Change them when you suspect them.
Bad wires will give plenty of warning before
they die. Misssing at idel and while cold,or on real
humid days. When that happens, don't wait, change
then ASAP. This assumes you check the rotar and cap
at regular intervals. I check mine the first week of April
and the last week of Oct., roughly every 6 months.
Terry