Have a look at the armor inside your jacket. Is the foam creasing? Is the hard plastic shell damaged? Have you had your jacket with viscoelastic protectors for longer than the suggested time? Is the velcro worn out? Is your waterproof jacket letting water in? If you answer yes to any or all of these questions, chances are it’s probably time for a change.
Ryan F9 also points to the reflective components of your jacket as an indicator. If they become dull and lose their capacity to reflect light and make you visible, you should change jacket.
The good news is that in some cases, if the fabric and the fasteners on your jacket are still in tip-top shape, you might be able to buy new pieces of armor instead of replacing the entire jacket. If the jacket is damaged, scratched or if the seams are coming undone, then you might want to consider doing a little switch-a-roo.