How many mm sacked can a spring be before it needs replaced?

[mx100]

04 Skidoo 800 calls for pr/yl spring, free sag 158mm. The spring is 151.8 mm. Can it still be used, or should it be replaced? How sacked is to much?

 

[mx100]

After standing the old spring next to a new spring, I realized 6 mm sag is a lot of free sag. We replaced it.

I also just heard from a clutch tuner that tests 1000's of spings, that a used spring (1000mi) can have the same freestanding height, but wont hold the same presure as a new spring. As cheep as these springs are, I found out it is just better to replace them.