Vitamin b complex caused me to break out in acne, after taking it for a year, how long will it take for my skin to improve? i've stopped taking it.

There's no telling: Trying to figure out what's causing one to have acne is futile. Ignore all "pop" ideas and start using topical benzoyl peroxide 1-2x daily, perhaps 5%. It's not pleasant but it's your best friend. If it doesn't clear you, your physician can add prescription strength Rx. My acne remitted when I was 50; yours will too, someday. Until then, manage it like the serious medical problem that it is.