Hey Dude,
It's like saying how many pieces of string do you need to reach the moon. You only need one if its long enough.
Time to draft a specification for functional and user requirements has dependencies on the POC for those respective areas. The problem is and you will find this a lot with consulting work, is that delays come in not from the developers side but from business. You will reschedule meetings for the same agenda on numerous occasions just to get the right people in the room to tell you what you need to know.
Lets make a caveat and say that everyone supporting the development of that dashboard has provided you will the right specification and data. Can you quantify how much more efficient and effective Person A would be compared to Person B? No you can't!
There is no exact science to providing a project time frame. There are techniques and certain assumptions that can be made, but what you need to ask yourself is why you are being asked.
More often than not, you need to set and meet the expectations of your customer. Build some 'fat' into your assumptions and always try and deliver within budget and time. If you are able to get the expectations right and know you have left enough time to potentially finish the project within budget, you will make your customer very happy
