As a frame of reference, where I was they expected fellows on call to read something on the order of four cross-sectionals an hour, or 10-15 plain films.
That was way faster than the average pace most junior residents read at, but I think it is way slower than what you would probably be expected to do in the working world.
I think four is a reasonable number to work towards - that means you get 15 minutes to read, dictate, proofread, and sign a CT. Obviously, some studies you can do way faster than that (cough negativedvt cough) and others may take much longer.
Another way to look at it is how many studies can you read in a day. A number for private practice I have seen come up frequently is 100 (mix of all kinds of exams).