At what stage of the project is it advisable to start testing?

Testing is advisable to start as early as possible in the project, preferably during the planning and requirements development stage. This approach is called "shift-left testing".

Early testing helps identify errors and discrepancies at the earliest stages, reducing the costs of fixing them.

For example, you can start by writing test cases and automated tests for modules immediately after their development, as well as reviewing requirements to prevent errors in specifications.