I believe that the Users Online setting only affects how DNN keeps track of how many users are online (how long a user is inactive before they are considered offline). I don't think that it will affect the session length, and I know that it won't affect how long you are remembered. What behavior are you seeing that makes you want to lengthen the session? The session should take care of itself under most circumstances.
The "remember me" length of time is set in the web.config.
<authentication mode="Forms">
<forms name=".DOTNETNUKE" protection="All" timeout="60" cookieless="UseCookies" />
</authentication>
The timeout attribute defines how many minutes that the authentication cookie is good for. This is 60 by default, which is why it seems that it never works. Here's the MSDN documentation on the forms element in the web.config,.