Using playframework 2.1.0, you can set the expiration for the default session cookie doing the following. There's a lot of articles whatnot about this, most of them though don't work. I did the following and it worked! Enjoy!
in application.conf, set the desired lifetime in seconds
Note that trying with 7d or any such number doesn't work.
Then issue a play clean compile to apply this change.
Then enjoy the new 10 day cookie.
To test it, I used Firefox with Firebug and the Firecookie plugin. Yes - even that doesn't work as advertised: to get the cookie tab in firebug, you need to install this new add-on, the Firecookie.
p.s. Update for play 2.3 and up - this is now a duration like 7d not just a number.