It must be whether someone else takes it. If you stay on line you keep the same IP for the length of the "lease". Like I say I think it is 24 hours
I released my IP, effectively going off line, when I was going away from my PC. I renewed on my return about 15 minutes later and got a different one. Presumably someone had taken the old one in the meantime.
So for anyone on NTL it would appear to be fairly straight forward. DHCP release, have a nice cup of tea and perhaps a cake. Then renew. With any luck you will get a different IP.