I am currently helping a new to Trumpet/Bugle older gentleman (retired) to play. We started last November, with the intent to have the Last Post for ANZAC Day = April. He had the Bb above the stave by March, and the Last Post under his belt for home play. We practiced only Bugle, so he could get used to the open partials, and understand tuning by lipping up and down, and long tones.
He has the High C pretty secure now, and has started on Bb Trumpet about a month ago. He has a decent tone, and he has a great ear. So I am sure that a High C can happen within a year.
I started on Bugle when I was 13, and started on trumpet before I was 15, and had a range of 11 open notes... we did not learn music to play, we just used numbers. I started to learn Trumpet at 14. Moving air, and projecting with a Bugle gave me all the basics I needed, but learning music, and correct trumpet playing both got refined with a teacher.
And we all struggle with that next partial - nothing comes easy.