The rough estimate is 6 to 12 months, but it depends on several factors, including:
The industry your business is in
The scale of the project
The backend processes and if you have them ready (you can only build the front-end part of the app with React)
The scale of blockchain research to be done for the project
The developers you hire, etc.
To get the final numbers, including not only the time but how much investment to spend on creating a React dApp, consider our Discovery Phase. Our business and tech analytics will evaluate your project and come up with your personal timeframe, the number of people needed, and the investments.