How long a publication takes to respond to submissions is something that we shouldn't get hung up on, but it's good to know for practical reasons. It can be useful to know if the wait is likely to be a month or if the market you're sending your story to usually takes closer to a year instead. Knowing the approximate wait time can help you plan a submission strategy for a piece, or keep you from worrying too much if you know you probably still have weeks or months ahead of you.
Duotrope, which now is a pay service, lets user submissions to their site measure approximate reporting times for each market. While these numbers shouldn't be taken as a hard rule, they do provide an estimate that's useful. I know many people stopped using Duotrope when it went to a pay service, and a smaller user base no doubt provides less accurate information. But for those willing to pay for the service, it's one way of seeing real-time reporting numbers, which are often very different from the publication's stated reporting time.
An option that I've used over the years is The Black Hole. Part of the Critters workshop (which now offers free workshops for a variety of genres and not just horror), The Black Hole lets you report submissions and responses to a variety of publications, most of them genre markets. Unfortunately, it seems that not nearly a fraction of the people who use Duotrope used The Black Hole, so many publications haven't had times reported in several months or even years. But if more people knew about the free tool and used it, the information would progressively get better over time.
If anyone knows of any other similar resources, please share them in the comments!