When developing websites with OSS, most likely that you're using the good eternal PHPMyAdmin for the DB interactions. One annoying thing about development, is that you rarely need the connections to the internal DB to be more secure than it would normally be on the server, or need just a minimal security as in case when you're working in a company which is protected by a good firewall. And why am I reminding this? Because during a work day you can return to PHPMyAdmin several [tens] of times and each time rediscover this annoying message which forces you to type again and again the username and the password, or at least, to click on the 'Go' button, or you know what? You know...
Find your config.default.php
with something similar to
Open the file, find the line which contains LoginCookieValidity
and set the value to something like:
$cfg['LoginCookieValidity'] = 54000;