While this type of question would ordinarily seem to be a simple inquiry, there are too many variables that come into play in any case to determine what a sentence will be based upon the few facts you list. The litany of aggravating and mitigating factors that can affect the sentence is not short. Consult with his lawyer, who can review all of the case facts in light of the aggravating and mitigating factors, and who would then be in a position to properly estimate the sentence. Good luck.
DISCLAIMER I do not practice law in your State. This answer is provided solely for informational purposes only. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.
If you are convicted of Robbery in the First Degree and you are a non-violent predicate felon, the judge can sentence you to between 8 years to 25 years. The prosecution can offer you something less on a "C" felony plea, but the judge would sentence you as a non-violent predicate felon as described above. This sentence is a determinate sentence (i.e., not a range of years) and would require post-release supervision as well.