My external IP address changed when I had to power off my router once, but under normal circumstances my IP doesn't change for months on end as long as I keep my router connected. I'm very happy about this, but I'm curious to know if it's likely to change at some point?
I had Virgin broadband a few years ago at another address and I remember it used to change the IP every month or so.
There are really only three things which will make your IP address change, change the Hub or switch it between modem and router mode, leave it off for long enough that the lease expires and the address is assigned to another user (and we are talking about leaving it off for over a week) or lastly, if and when VM carry out any reconfiguration work which involves moving your connection to a different 'segment' of the network which is done to better load-balance the connections.
There's no real way of knowing when the latter will happen but it tends to not be very frequent, it's not uncommon to keep the same WAN IP address for years!