Unlike eBay, Amazon API does not provide a real-time listing processing. It uses a queue system, i.e. all requests are processed on a “first come, first served” base. The response time of Amazon servers depends on how loaded Amazon system is at the moment of request. Normally, it takes up to two hours. Yet, it may be longer if the Amazon system has some temporary issues. Your Listing Product will have the "In Progress" status until Amazon API returns the response to M2E Pro.
It does not matter how many products you are listing — it takes the same time until Amazon returns the response. We advise applying the actions e.g List/Revise/Stop/Relist to a big group of products at once to save your time.