At any time you can register for Final but before to filling of your final examination form. I.e as on the date of the filling of examination form you should be final registered holder. So better to register before 6 months prior to your final attempt. As well as in case you completed both groups of IPCC and now registering for Articleship then you should be register for final along with the articleship registration.
ok, i have cleared both groups of IPCC but my oly worry is if i register now i will be getting very outdated study mateial cos my final attempt is after two and half years