Edit your config.inc.php file (in my Ubuntu 12.04 it was located at /etc/phpmyadmin/config.inc.php) and add the following line (where X is the amount of seconds the session will last):
$cfg['LoginCookieValidity'] = X;
In my development machine I have 36000 that is 10 hours (all work day) but you should never do this in a production environment.
You may also have to set the same value on session.gc_maxlifetime in your php.ini file because it will probably be smaller than LoginCookieValidity value you set before (it’s default value is 1440).
session.gc_maxlifetime = 36000
Ref: http://venutip.com/content/increase-phpmyadmin-session-timeout