I agree with @Bonxie that it depends on your pc and game settings. My pc is scratch built Intel i5-4670K CPU running at 3.4 ghz, a 250 GB SSD, 16 GB of system memory and a nVidia GTX780 with 3 GB of dedicated memory. I run the game in 64 bit mode.
I was up to 12 GB of mods/custom content (mostly custom content) but then my game started pausing every 5 minutes so I deleted the entire game folder and started over. I did save everything before doing deleting and then started adding back my mods/cc slowly and check to make sure I wasn't adding back the problem. A week later and I still have not had the same problem, so I assume it was a glitched game save.
I am now back to 9.6 GB of mods or 6,400 files in 202 folders and it takes about 3 minutes to load my game. Once loaded, my games runs smoothly (`60 fps with dips into the 30s).
And, of course, I will be adding more, as I am addicted to new things!