I love this stuff, i use it on my arms and sometimes belly hair. I have dark hair and i usually leave it on for 10 minutes and they're blonde by then...though some of them are orange sometimes. I'd reccommend you leave it on for about 15-20, just to be sure, speaically coz brow hair is much thicker than arm or belly hair. I'm pretty sure it's safe to leave on for that long but i don't know.
You should do a patch test to find out what works for you, and to find out that hopefully you're not allergic or anything.
Oh and do it in a well-ventilated area - this stuff has some serious potential to get you high lol