9 AM to 9 PM 6 days a week. It's a "how long do we work" shorthand, like 955 (9 AM to 5 PM 5 days a week) or 864 (8 AM to 6 PM 4 days a week).

It's the first hyperlink in the article, and the first Google result for "996".

https://en.wikipedia.org/wiki/996_working_hour_system