I have had a thread running on the short WAN lease times used by TalkTalk and they have suggested that this 15 minute (900 second) is normal for ISPs to preserve IPv4 addresses. I really cannot believe that most ISPs have such short lease timings, if a router behaves correctly it will renew the lease at 50% of the lease time which in this case means every 7.5 minutes.
Can people here please take a look at their routers to see if they can ascertain their lease time for their IPs? I'd like to hear from as many as possible including other TT users. For ZyXEL users the way to find this is to use the router log and set to record WAN DHCP entries. I'm not sure if other routers may show this on a status page somewhere, otherwise a log seems to be the way to go to view this.
Stuart