^ since your hair is natural, it's probably not super porous(sometimes bleached hair has holes that hold dye better but not necessarily evenly/in a pretty way).
volume 10 sounds about right for you. since you were blonde only going light brown, it's not a huge change. the higher the volume the more hydrogen peroxide to open up the hair shaft to soak up the color. level 7 color sounds dark enough as well. it just might have to do with the brand of dyes they are using. when i did light to medium blonde, i think staying on one month is not bad considering my hair was completely bleached, it still faded pretty. i have friends and family who have gone to other salons to dye their darker and it fades way faster than one month so i understand your frustration.
you should definitely have a chat with her when you are there next time before she starts mixing colors for you. i don't know if her plan is to redye with the same color to saturate more or go a little darker hoping it will stay on more. like even if its 'too dark at first', it may fade nicer and not back to blonde so quickly. that's why i hate it whenever i darkened my hair. it always turns out darker than what i want but that usually fades to the 'ideal color' and in turn it lasts longer.