Although generative AI is rapidly becoming a core workplace tool, empirical evidence on usage patterns and productivity in real organizational settings remains scarce. Using a survey of 2,992 wage workers in South Korea, this study finds that 56.3% use generative AI at least weekly, with notable disparities by age, occupation, income, and firm size. Workers report average time savings of 17.6%, yet larger self-reported time savings are observed only when usage is combined with prompt engineering capability. These associations should be interpreted as correlational rather than causal, given the cross-sectional self-reported survey design. Gains also vary by task type, being pronounced for document writing but negligible for idea generation. These findings suggest policies should prioritize capability development, workflow redesign, and governance frameworks to mitigate adoption disparities.