PIP award length appeal - any guidelines in law ?

Hello all,
I'm appealing my PIP as I've only been awarded standard daily living. However, as I'm appealing, I will also ask for longer award. The DWP gave me 3 years, reviewed in 2. I've since heard of housebound ME sufferers getting 5 years from DWP.
1 - Are there any guidelines in law which the tribunal will follow in deciding the award length ? I can only find a vague statement about 5-10 year awards if the condition is unlikely to change.
2 - I didn't contest the award length at the MR : will that prevent me from raising it at the appeal ?
Thanykou for your help.

Hello Gordon,

I was awarded enhanced daily living and mobility for ten years, by the DWP at the last stage before the hearing was scheduled.

I detailed the effect that repeated assessments have on me, but I also had plenty of evidence.

Sadie