Plugin for limiting user registration based on ip with expiry period?

Despite that this approach may be flawed by the fact that it can be by-passed using proxies, here is a simplistic (yet untested) approach, which you would need to improve upon but would give you the foundation for achieving your desired goal.

The process as I see it:

• filter user registerations on the pre_user_login or pre_user_nicename hooks
• check database to see if IP exists in a time-limited blacklist
• if IP exists within range, reject registration with custom error message
• if IP does not exist within range, add the IP to the time-limited blacklist
• rinse and repeat for each registration attempt

Example:

function filter_user_registration_ip($user_nicename) {

    $ip        = $_SERVER['REMOTE_ADDR'];                    //get current IP address
    $time      = time();                                     //get current timestamp
    $blacklist = get_option('user_ip_blacklist') ?: array(); //get IP blacklist

    /*
     * If IP is an array key found on the resulting $blacklist array
     * run a differential of the 
     * 
     */
    if ( array_key_exists($ip, $blacklist) ) {

        /*
         * Find the difference between the current timestamp and the timestamp at which
         * the IP was stored in the database converted into hours.
         */
        $diff_in_hours = ($time - $blacklist[$ip]) / 60 / 60;


        if ( $diff_in_hours < 24 ) {

            /*
             * If the difference is less than 24 hours, block the registration attempt
             * and do not reset or overwrite the timestamp already stored against the
             * current IP address.
             */
            wp_die('Your IP is temporarily blocked from registering an account');
        }

    }    

    /*
     * If the IP address does not exist, add it to the array of blacklisted IPs with
     * the current timestamp (now).
     *
     * Or if the IP address exists but is greater than 24 hours in difference between
     * the original stored timestamp and the current timestamp, add it to the array
     * of blacklisted IPs.
     */
    $blacklist[$ip] = $time;
    update_option('user_ip_blacklist', $blacklist);      

    return $user_nicename;

}

add_filter('pre_user_nicename', 'filter_user_registration_ip', 10, 1);

Notes:

• The above code is untested and may contain errors.
• The approach to retrieving the current user IP is not fool proof.
• The array of IPs will grow exponentially overtime, you will need to prune the array periodically.