Looks like an old pool vehicle at a previous job. I would clean it out on a Friday on the way back to the office. I'd grab the same vehicle less than a week later and it would be full again.
Car is still pretty new, so I try to wash it at least every 3-4 weeks. I have a few things behind my seat like an empty laptop bag and a few power adapters, things that I guess I should return to storage. Vacuuming is the one thing I'm slacking on. Bought my car in either May or June and still haven't vacuumed.