Whats a reasonable timeframe to expect solid code contributions? These employees have 3-8 years of experience.
- 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.
Experience doesn't need to be an indicator for productivity. Also often you can find programmers who seem not productive at all, but without their contribution the whole project might fail.
The best thing to do is to set your milestones, and put trust in the people you hired. Then check your progress regulary. Talk and listen to your employees. Be trustworthy yourself.
You don't want to know how much code a new hire can put out in a month. It doesn't matter. You want to know weather you reach your business goals. So communication, trust and transparency will improve every contribution of your engineers.