Should the T and L phase of a well tuned ETL report process take the same amount of time regardless of the time bucket width and data size?

For an ETL reporting system, is it normal for the total execution time for a pull of 15 minutes with no data to be similar to a 24 hour pull with data? ------------- I had expected that the total time for ETL when there is NO data would be shorter but that is not the situation between a 15 minute and 24 hour pull. But I must confess that I know nothing about the internals of the T and L phases within a report server. Can someone shed light on whether the T and L phases are typically fixed in duration (up to a point)?