DVLA helpfully send you a letter to remind you well in advance (e.g. Aug for an Oct expiry). And handily they can link it up with your passport so no need for new pics or fannying about at the Post Office.
HOWEVER: don't do what I did and deal with it immediately so as not to forget, because they won't add on the remaining few months validity to the new one.
I would have waited till the last minute if I'd known that, especially as they say you can continue to drive before your new photocard arrives, as long as you've submitted online and got the references etc 