Long lasting box hair dye? Help?

I have a problem. It has a name, we call it "gray" 😩😭 I'm 28 years old and this shit isn't playing no games! My natural hair color is "dirty blonde" or "dishwater blonde" it looks terrible on me, I am NOT meant to be blonde. I've been dying my hair to a medium/dark brown since I was 16, that's been going great, however recently I find my grays don't stay covered but maybe 2 weeks after dying it. I usually dye my hair every 3 months, I sure as hell am not dying it every 2 weeks! I need help ladies! Best hair dye for gray coverage? Thanks!!!