Textbox textchanged Event

hi!
i am writing code to send messages to mobile phones. so i am taking 3 textboxes. first textbox is for writing a message and second textbox is display a howmany characters left and third textbox is, default text is "1" if user is reach limit of characters(150), that time third textbox display text  is "2".
 
in my program user enter number of characters and charcters for one message is "150". if user cross the limit of chacters that time "2" messages  send, upto use "5" messages.
 
my aim is if second textbox value is 151 then 3rd textbox display "2" and
second textbox value is "301" this time 3rd textbox disply "3". similarly.
 
how to write the code for do it. please give a code very urgent.
i write code for when user enter text, that characters length is displayed in second textbox.  
Thanks.