when should i replace my spark plugs?

Plugs

I wouldn't go ofer 30k-50k. The car will run OK after that but it puts a strain on the ignition system. Cheap insurance.

Good question, I have been thinking about the same thing.
I have a 96 GT with about 40k miles (or 62,000 KM's) on it and just due to age I may change the plugs this spring. The only thing that has stopped me in the past is it appears to be a Bi-ch of a job. When they designed these 4.6's some things seem a bit more difficult than they should be (another example is the da-m oil filter, try and get that off without making a mess).