Hello James, your best indicator might be the taste. Generally speaking, once the taste of your vapor starts to be affected (meaning that you can feel burnt, unpleasant taste when vaping) it is time to change your coil.

The time it takes to use a coil depends not only on the frequency of your use but also on the quality of your wax.

It is better to change the coils more often than less, to make sure you get the best performance and taste out of your concentrate.

Welcome to the VapeFuse Community James. SupercellV is spot on, I also tend to think that taste is your best indicator.

Obviously you'll also see the coil getting darker and darker, and eventually it reaches a black color. I suggest switching to a new coil before it gets to such a black color.

The Headbanger coils are quite affordable and it makes sense changing them regularly.

I can't post an image in this comment, but I'll post a couple of pictures of a Headbanger coil that started to reach the black color and definitely needs to be replaced. I hope this helps and let us know if you have any more questions.