Should the T and L phase of a well tuned ETL report process take the same amount of time regardless of data size?
There's nothing specifically quantifiable about the abstract concepts of Transform and Load, only their concrete implementations are measurable. To be able to comment on your case, we'd need to know specifically what your Transformation and Loading processes are actually doing. Obviously some Transformations can take multitudes longer than others.
But generally speaking, the amount of data being processed definitely should affect the overall runtime of an ETL process. If there's a significant difference in the amount of data between a 24 hour timeframe vs a 15 minute timeframe, yet your ETL process is executing at roughly the same average runtime for both cases, then something is definitely suspect, and that's not normal.
Even if an index scan is occuring in both cases, if there's a significant difference in the amount of data, the total runtime should certainly reflect it. The runtime of an index scan is linear (generally speaking) based on the number of rows in the index.