What is the blood ejection speed outward from human body vein?
Ah, I'm afraid there are a near infinite number of variables that will make a general answer impossible.  Factors such as: age, sex, weight, which vein, additional damage, heart condition, cholesterol level, body position, body temperature, and how the vein was cut and exposed will all lead to different answers.

A general answer from someone with lots of trauma experience, not very far.  Rarely if ever greater than 10cm.  Veins just don't have the pressure that arteries do.