If you are on a regular diet and then stop ingesting gluten, it can take six months for your body to be totally clear of it.

what if you're on a gluten free diet and accidentally eat a small amount of gluten? How long does it take for the gluten to leave the body then?

My understanding is that it can take anywhere up to 2 days for a small amount of gluten to leave the body, but the effects it may cause to your body can last longer. It would depend how susceptible you are. Some people react more violently to small amounts of gluten than others. My son vomits when he accidentally eats gluten, but some people do not have any outward reaction to small amounts. Of course it can still be causing havoc with your insides unbeknownst to you.