How do I keep a user logged into my site for months?

So you actually want like a "Remember me on this computer" option? This is actually unrelated to OpenID part. Here's a language-agnostic way how you can do it:

• First create a DB table with at least cookie_id and user_id columns. If necessary also add a cookie_ttl and ip_lock. The column names speaks for itself I guess.

• On first-time login (if necessary only with the "Remember me" option checked), generate a long, unique, hard-to-guess key (which is in no way related to the user) which represents the cookie_id and store this in the DB along with the user_id. Store the cookie_id as cookie value of a cookie with known cookie name, e.g. remember. Give the cookie a long lifetime, e.g. one year.

• On every request, check if the user is logged in. If not, then check the cookie value cookie_id associated with the cookie name remember. If it is there and it is valid according the DB, then automagically login the user associated with the user_id and postpone the cookie age again and if any, also the cookie_ttl in DB.

In Java/JSP/Servlet terms, make use of HttpServletResponse#addCookie() to add a cookie and HttpServletRequest#getCookies() to get cookies. You can do all the first-time checking in a Filter which listens on the desired recources, e.g. /* or maybe a bit more restricted.

With regard to sessions, you don't need it here. It has a shorter lifetime than you need. Only use it to put the logged-in user or the "found" user when it has a valid remember cookie. This way the Filter can just check its presence in the session and then don't need to check the cookies everytime.

It's after all fairly straight forward. Good luck.

See also:

• How to implement "Stay Logged In" when user login in to the web application
• How do servlets work? Instantiation, sessions, shared variables and multithreading

Well, the original reason I chose OpenID was so someone else could handle as much of the implementation and security of authentication for me.

After looking into OpenID more, it appears there is something called an "Immediate Request" (http://openid.net/specs/openid-authentication-2_0.html#anchor28).

When requesting authentication, the Relying Party MAY request that the OP not interact with the end user. In this case the OP MUST respond immediately with either an assertion that authentication is successful, or a response indicating that the request cannot be completed without further user interaction.

Because of this I think I could just store the user's openID url in the cookie, and use an immediate request to see if the user is authenticated or not. This way I don't have to do anything with my database, or implement any logic for preventing session hijacking of the long-lived cookie.

This method of doing it seems to be the way OpenID suggests to do it with their Relying Party Best Practices document.