Yes, I know fixed_ip_disassociate_timeout can disassociate ip address, but this method could run when host value in networks DB.
But, When we make a network, host value didn't put any value, So default host value is NULL.
For example)
# nova-manage network create test_network 10.102.0.0/24

mysql>select * from networks\G;
*************************** 2. row ***************************
         created_at: 2011-12-09 05:20:13
         updated_at: NULL
         deleted_at: NULL
            deleted: 0
                 id: 2
           injected: 0
               cidr: 10.102.0.0/24
            netmask: 255.255.255.0
             bridge: br100
            gateway: 10.102.0.1
          broadcast: 10.102.0.255
               dns1: 8.8.4.4
               vlan: NULL
 vpn_public_address: NULL
    vpn_public_port: NULL
vpn_private_address: NULL
         dhcp_start: 10.102.0.2
         project_id: NULL
               host: NULL
            cidr_v6: NULL
         gateway_v6: NULL
              label: test_network
         netmask_v6: NULL
   bridge_interface: bond1
         multi_host: 1
               dns2: NULL
               uuid: 07774f63-df94-4ddb-9274-800ff8e5ae20
           priority: NULL



because host value is null,  fixed_ip_disassociate_timeout method didn't run properly.

this is source code of network.manager.periodic_tasks
 509     def periodic_tasks(self, context=None):
 510         """Tasks to be run at a periodic interval."""
 511         super(NetworkManager, self).periodic_tasks(context)
 512         print('timeout_fixed_ips: {}').format(self.timeout_fixed_ips)
 513         if self.timeout_fixed_ips:
 514             now = utils.utcnow()
 515             timeout = FLAGS.fixed_ip_disassociate_timeout
 516             time = now - datetime.timedelta(seconds=timeout)
 517             num = self.db.fixed_ip_disassociate_all_by_timeout(context,
 518                                                                self.host,
 519                                                                time)

maybe nova-network send self.host and time value through self.db.fixed_ip_disassociate_all_by_timeout.

but In database, host is null so 

self.host != host value in db

db.sqlalchemy.api.fixed_ip_disassociate_all_by_timeout could run properly when 

self.host == host value in db

this is source code of db.sqlalchemy.api.fixed_ip_disassociate_all_by_timeout
 801 @require_admin_context
 802 def fixed_ip_disassociate_all_by_timeout(_context, host, time):
 803     session = get_session()
 804     inner_q = session.query(models.Network.id).\
 805                       filter_by(host=host).\
 806                       subquery()
 807     result = session.query(models.FixedIp).\
 808                      filter(models.FixedIp.network_id.in_(inner_q)).\
 809                      filter(models.FixedIp.updated_at < time).\
 810                      filter(models.FixedIp.instance_id != None).\
 811                      filter_by(allocated=False).\
 812                      update({'instance_id': None,
 813                              'leased': False,
 814                              'updated_at': utils.utcnow()},
 815                              synchronize_session='fetch')


So, I think there is some host value when create a new network to DB.
OR db.sqlalchemy.api.fixed_ip_disassociate_all_by_timeout don't check host value in DB.