Hi Cazza74105
I can't tell you how long it will take. Hopefully, as you have been his appointee before, it will be a quick process. Check out the government advice on this
www.gov.uk/become-appointee-for-someone-claiming-benefits.
I know the DLA people have said - do it on the first call for PIP, but I would ring the free PIP line and see whether you can start the process now. If they say wait until the first call you have your answer.
BIS