[franzjoy]

This question cannot be answered easily. It depends on so many details... Are the original authors still there to guide new employees? Is the overall design clear and simple?

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.