I used lush henna for 2 years but stopped in the end because I found it too much hassle.
The colour will lose its intensity but it won't fade out completely. Chemical hair dyes only coat the hair but henna actually penetrates the shaft so it's near impossible to get rid of.
This is going to sound really stupid but did you definitely buy the right colour? Just they all look very similar and I did once accidentally grab the wrong one and not realise til I'd put it on.
My hair is naturally a mousey brown and I used maron mostly and rouge once. Maron gave me a lovely dark brown with a red tint. Rouge made me very ginger (my sister said I looked like a red setter). Even with light brown I'm not sure why it would go ginger, they say lighter hair is more likely to go red (which is why it's not recommended for blonde or grey hair) but it shouldn't happen to this degree with brown hair.
It'd help to see photos of before and after but from my experience if you have lighter hair it's a nightmare to get rid of. With darker hair you can use a darker henna and then the new growth blends in quite nicely. If your natural colour isn't close to the henna colour it's more difficult. Because it's inside the hair the colour never really goes completely, it just reduces in intensity but obviously if your natural colour is nowhere near the original colour than you'll still get awful roots. In the end I dyed over mine using chemical dyes. Seeing as it's not black henna it's safe to do but obviously do a strand test first. Dyeing over it worked but even a year after when I used that colour b4 stuff to see what I was working with the henna was still very obvious. I wouldn't recommend trying to bleach it out. I did find a blog post where a woman had managed it but it took repeated bleaching and she had very thick hair which was so damaged from all the bleaching that lots fell out. If you don't want to keep the ginger brun might be a closer match to your own hair?
Sorry I didn't have better news, henna makes your hair so shiny and gorgeous but it's not easy to get rid of.