Unlike eBay, Amazon API does not provide real-time listing processing. It uses a queue system, i.e. all requests are processed on a “first-come, first-served” base. Your Listing Product will have the "In Progress" status until Amazon API returns the response to M2E Pro.

The response time of Amazon servers depends on how loaded the Amazon system is at the moment of request. Normally, listing processing takes up to two hours. Yet, it may be longer if the Amazon system has some temporary issues. 

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.