Many factors influence when you can schedule concurrent copy operations.
Example: The structure of your overnight batch processing determines at what stages in the processing you can use concurrent copy. Similarly, availability of tape drives may restrict the intervals during which running a concurrent copy operation is feasible.
If other considerations are not a factor, use concurrent copy at times of lowest activity, especially lowest update activity. It may, however, be beneficial to use concurrent copy to back up data even during periods of higher I/O activity. In some cases, ensuring data availability may be more important than preserving levels of application performance.