php session expiry

Dear Sir,
I am facing endless problem, Pls help me.
My prob is... (note I am not expert...Just i am started php)
I have php page, with out having any session expiry time limit, just i used session start only.
nature of my page is required more time with out any activity, so that i did not specified time for expire....
But after 20 min (aprox) when my client clicks submit button, It redirected to index page ie due to session expired (i think so)

I used lot of coding to solve that problem, My clients firing me to extend expiry time limit... But there is no use.

In my initial study it may be problem due to browser, especially any version of IE and earlier version Chrome & FF (but i am not sure)...

I used following codes, but no use, Before session start.

$config['sess_match_useragent']    = FALSE;
	ini_set("session.gc_maxlifetime", "3600"); // value in seconds  this is 1 hr
	ini_set("session.cookie_lifetime","3600");

Any one can help me.. Please. Thanks for spending time for me.