This is such a difficult answer. For every girl who's having a bad month, there's another that's having a superb one. It all boils down to luck and hard work. Granted, there are times when the entire site is having lower views, but even during these times, some girls don't notice it at all. There are some threads in MO that might help you gauge the overall feel for if a site is slow on a certain day (look for "No or Go" in Models Only), but threads like these only show reviews from the girls that post. Personally, I never read nor post in them, because when others are saying they're having super-slow periods, I might be having the best month ever. Or other girls are having the tokies rolling in, and I've only got tumbleweeds in my chat.
There will ALWAYS be girls complaining about the "winter slump," "summer slump," that the site is slow because of holidays/sports/school/vacations/taxes/whatever. In the end, don't let it affect you. Keep consistent hours, stay positive (especially while on cam/social media), and work work work. That's the only way to get new viewers and help ensure that you won't have to worry about the "slumps."
To quote Dr. Seuss: "And when you're in a Slump,
you're not in for much fun.
Un-slumping yourself
is not easily done."
However, it can be done. So keep your head up, and keep trying!