PIP Appeal

I submitted an appeal to the HM Courts and Tribunal Service on 3rd December 2018 in relation to my Daughter's PIP application. I have received a letter from the tribunal service acknowledging that they have received my appeal and that they have asked the DWP to prepare a written response to it - can anybody tell me how long do the DWP have to submit this, I'm sure I have read somewhere 28 days. Grateful for any information.

I submitted extra evidence with my appeal, could the Dwp change the decision?

Thanks

I am still awaiting the DWP's response to our appeal - have been in contact with the HM Courts and Tribunal Service and they told me that they would send the DWP a reminder asking them for their response and that they would give them 7 working days to reply - this was one the 14th January. To date I've received nothing - can anybody advise - do I get in touch with the Tribunal Service or do I just wait? We have another appointment at GOSH on Thursday with my daughter and I've had to ask family for help with train fares. Any advice please?