Your commissioning editor should tell you when this process begins and how long they expect it to take. Remember that your publisher is relying on busy academics to take time out of their other commitments in order to read and report on your material, so reports often come in later than hoped. If you hear nothing after the indicated timeframe has passed, it’s fine to drop your editor an email to ask if they have an update – they should be able to tell you when the reports are due. Once the reports are in, your editor will be keen to get them to you.