Hi,
I am an expert Java Professional and more than a decade I am developing and delivering applications using Core Java, Servlets, JSP, Struts, Spring Framework, Restful Services, JSON, MYSQL, Oracle and MongoDB.
Since you did not share the code, how big the application size is and quality of the code, I could not give the right estimation.
Always estimation should happen based on the exact details, then only it will suit for both the parties.
Let me know, if you are interested, we will discuss and will start the work by today / tomorrow itself.
Thanks & Regards
Java Developer