The timeline will depend upon certain criteria such as requirements, complexity, available resources, expertise, etc. So, it’s quite hard to predict an actual range for every web app development project.
It entirely depends on the scope of the project. However, on an average, it takes 4-5 months to build a web app from scratch. Else, if you’ve all the modules ready for the web app, it might take a period of 3-4 months.
By communicating your requirements precisely to your web app development partners, you can possibly optimize the time period.