In addition to all the things others have said, it also depends on where you are (because the frequency of types of systems vary: lots more black holes near the core, lots more whire dwarfs high above or below the galactic plane, and so on. On top of that, you have to consider how you choose the systems to explore. Most CMDRs probably use the automatic route finding in the galaxy map. That totally ignores your filters (which is why I don't use it) so you tend to go through the average collection of systems. Which includes an awful lots of M4 VAs with no planets, or brown dwarfs with a string of icy planets, and so on. I plot my routes manually, one jump at a time (since FD have neglected to give us tools to plot more than one jump ahead). That means that I hardly ever visit those brown dwarfs, etc., and in consequence my average per system will be higher than someone who uses the route plotter, even with identical in-system exploration processes, and covering the same route.
As others have implied, as asked, the question can only sensibly be answered with something like 'it depends'.