ESA/PIP Severe Disability Premium Backpay?

I've seen posted elsewhere over the last 2 days that once the IS10 form is with a decision maker they have 6 days to make a decision is that correct?

Mine took 24 days from filling out the form over the phone to receiving the money, and that was in January or February (in case that helps).

Been told because the form was done over the phone they have 6 days to make a decision but like Gordon has said that isn't a legal requirement. Main thing that worries me that I won't get it is the underlying entitlement that my parent gets for me. But a few of the call handlers at ESA have said that shouldn't cause any issues.

I'm on income related ESA since 2013
I'm on PIP daily living and mobility since 2014
The parent/guardian who I live with has been on Attendance Allowance since September 2016 and gets an underlying entitlement to Carers Allowance which just adds a premium onto his Pension Credit

Surely I have all the requirements for SDP hopefully the decision goes in my favour.

Just received a phone call from the DWP DM awared SDP from 2016-29th May 2019 backpay in the region of £8900 which seem's little high. She did have to do some digging around because there systems were showing I was only entitled from 2018, glad its sorted now Thanks Gordon for your help and input