Err on the conservative side and do not let it sit more than 2 or 3 minutes before washing it out. If you follow the instructions and wait 5 minutes your whiskers will look purplish-black. Don't waste time with the allergy test either.
It is hard to nail it every time. Worst case scenario, providing you don't let the gel sit too long, you will at least dull down the white.
About a year ago my chin whiskers turned white, making me look like one of those dogs that looks like an old man. A few greys would not have been a big deal but white really looks like [censored]. Careful use of this stuff solved the problem.
Unless you are a cop, I hope you have some other facial hair along with that mustache.