How do we calculate node completion time

Which nodes do you mean? User input tasks? You can get this using process reports.

If you just want to know the completion time of each node and not using it anywhere then you can get this detail in Process details of an instance.

You can get this information from a task report.

You can use the tp!starttime and fn!task_completion_time() function to get the start and completion times. From there you can calculate the duration by subtracting the start time from the completion time.