Automatically calling method when textbox length is 10.

i am new to web development so please suggest me how to implement this requirement. my requirement is implementing results when user enter 1o digits in text box then call automatically  one method without clicking any button and also text changed event , that method getting data from database .i tried with text changed event but it's fire when focus leave in text box, i don't want like that  i want while focus on text box only it will call that method.

thank you in advance.