How do I tell the customer of the project how long it will take to complete the development of what he wants before starting the project?

Question

Project management

I am a beginner project manager and would like to know how to do it right in this situation

Answer 1

Fix the business requirements, make a system project for these business requirements for a separate cost, and the development cost will already be clear from the system project.

Answer 2

There is the concept of a pre-project survey, when requirements are collected and the time-resources-budget is roughly estimated. For typical projects, you can usually ask more experienced colleagues for an approximate price order or simply look for the cost of the project on the websites of integrator companies on the Internet, something unique is rarely found in work, there are probably already implemented examples.
And before showing the assessment to the customer - multiply the assessment by 3 (both in terms of time and budget), this is a conditional coefficient "if something is underestimated", derived empirically for more than 10 years of work on a variety of projects.