There is a problem with storing too many files in a single directory as the directory itself gets fragmented and performance starts to suffer. There are indications that this starts at lower counts but i start to see it when directories have more than about 1000 files. If you were to store one file every hour this would start somewhere around 40 days in.

Of course, the performance of directories full of files highly depends on the filesystem used. Caching (i.e. free RAM) and tuning of filesystem and operating system can influence the performance drastically, as can disk performance (compare an ancient harddisk with a high end SSD).

Alexander