I had a similar issue. Users came to me saying they were getting disconnected after about 30 minutes. if you edit the WLAN there is an option for "Session Timeout(secs)" and by default it was 1800 (30 minutes). We set it to 36000 (10 hours) so that it would last the day and have not head of any issues since.
I believe it can also be set for 0 which is the same as never timing out but we didn't want something to linger on there forever if it wasn't needed. Hope that helps.