i bought weapon, focus, chest and waist for U. parts and from crate i received head, boots and ear.... i don't count how many crates i opened but i think every 10 +/- crates contain legendary piece of gear.
And i opened a lot of crates considering that everytime i win GSF i usually have one crate ready in my inventory. And yes i'm a pretty succesfull in winning.
But as i said, i don't care if legendary item drops me in 10 crates or in 20....