Your back pay will be from the time you submitted the most current claim. Although, the temporary will start from right after your surgery. I don't know how long this will last because I haven't had back surgery. I have had knee replacement surgery and that temporary 100% lasts for 13 months. Then they go back and do a new exam and determine how much you permanent for that particular disability is and add that to your combined disability, (using VA math of course).