I depends on a lot of factors:
- How complex is your codebase? Is it a pretty standard CRUD app, or does it have extra components like machine learning or complex image rendering? Does it need to interact with services outside your company?
- Is your code easy to read, or a mess? Are there unit tests?
- Is there documentation about the system architecture and functionality (or people who can spend time providing a system overview), or do the new developers have to figure it all out on their own?
- How much specific domain knowledge would developers need to pick up to work on the application? E.g., would they need to know all about banking or HIPAA regulations to be able to write code that won't break laws?
Depending on the answers to these questions, the answer could be anywhere from days to months.