As Jillian said, when it thins, and you have spots that no longer cover like it used to.
I have a lot of wigs, most of which are synthetic. You can soften the fibers by soaking them in fabric softener (Like Downy or Woolite). They tangle because the fibers are getting rough from combing and product residue. Give them a gentle wash, then thoroughly rinse in cool water. Add just a tiny bit of fabric softener to clean cool water and let it soak for 15 minutes. Never swish the wig while washing or soaking. There are detangling conditioners made for synthetic wigs. They are a big help.
Allow to drip dry. Never use a brush. Comb with a wide-tooth comb after it is dry. Start at the ends and work your way to the roots. Be gentle, or you will pull fibers out of the cap. The downside of fabric softener is that it often has a fragrance. I don't like my hair smelling like laundry, so I air-dry it for a few days before wearing it.
If it still continues to tangle, the fibers may be worn or damaged, and then it is time to say goodbye. Or, if you are like me and really love it, and just can't let go, get a stylish wig stand and display it.