At what point should the novice player begin work on Kopprasch?

I started Kopprasch my 5th year of horn (10th grade). To fully appreciate the exercises, you need at least a little musical instruction outside practicing by yourself. Taking one-on-one lessons (at least for a couple of weeks if you're not comfortable committing long-term) would be a good idea so that your teacher can gauge where you stand and direct you toward appropriate repertoire. Otherwise, I think high school is an appropriate age to start Kopprasch.

Thanks Fredrick,
Thanks for the response.
I've started the horn in my my 50's...way past high school! I took weekly lessons until my instructor took a job with another orchestra and moved away, so I had lessons for a year and have been on my own for about 8 months. I'm just about to finish the intermediate Rubank horn method book and will go direct to the advanced books. Guess I should have provided a little more background information before posing my question!

I think you'd be good to start Kopprasch, then. It's very standard practice repertoire. Sorry to assume your age.

If you haven't invested in a recording device to practice with yet, that should be something you do as you're on your own. That way you can critique your playing from a listener's perspective and more easily compare to other recordings you find.