I currently have poor internet for downloading games or anything large (i.e. around 150 KB/s maximum download rate).
When I subscribed to a very large Skyrim mod (SMIM) via the Workshop, the mod never got downloaded.
After an hour of playing Skyrim without it, waiting for Skyrim to sync and the works, it never started downloading.
Data Files on Skyrim's launcher showed clear lack of any SMIM .esp files and refused to acknowledge that SMIM wasn't downloaded or installed.
Eventually, I had to unsubscribe on the workshop and download it via the Skyrim Nexus.
I don't want to do this, because I like how easy the Workshop is, especially since I have multiple computers that I play Skyrim on.
This brings me to my question: When does Steam Workshop mods download/sync? Can I force-sync Workshop mods?
Note: The Skyrim launcher didn't work for syncing mods. As a matter of fact, it said I was "All up to date!"
edit: This question is under the assumtion that all Steam workshops are managed the same through the Steam client.