Hello there,
So far I've written a code for two such assignments using flex, bison and C programming. In both cases, the code was written for lexical, syntax, and semantic analysis along with the interpreter. By interpreter I mean expression parsing and execution, as you asked in the description.
As I also have a good background in Python, I think I can help you in your assignment also. The only problem, however is, since I'm about to complete the second assignment, which seems to take nearly 2 days, I'll not be able to start working on your task immediately.
Also, the actual number of days required to complete your assignment really depends upon the code you've developed so far and the level of details in the specification. Though in most cases it will not take more than 3-4 days.