I have made a kind of a box out of brick for the structure of my forge and I was wondering how thick i should make the refactory cement layer. I also need to know how to get the cement i already have to dry. It's about a half inch layer in the base that still forms depressions when i poke it, and it's been that way for well over a week.

You posted this in the solid fuel forges section, so I have to assume that's what you intend to use for fuel.  You gave us very little information to work with.  If you are using solid fuel then I have to wonder why you are using cement, refractory or otherwise.  I'd suggest taking some time to read through the pinned and/or sticky topics regarding solid fuel forges and I think you'll find answers to many of the questions you may have.

May I point out asking us about the cement you used without giving us any of the details---what type, how you mixed it, etc.  is rather like me telling you to just read my mind for the answer...

It would also have been far better to tag this latest question on the end of the first thread you started as it refers to the same forge build, then we would have known what you'd already been advised to do and have more of an idea of what you have done, did you actually follow any of the advise previously provided?