Typically, I charge for services on the basis of the word-count of your script rather than the duration of the final audio. For example; for a commercial, I might charge for a read of 80 words rather than for a 30 second audio file.
The reason for this is simple: it’s fair! The amount that can be read in a 30 second spot (for example) varies depending on the speed of the read. A slow read may only need 70 words (or maybe even fewer) whereas a fast read could need up-to 90 words.
I believe that in this example case of two clients asking for reads of 30 seconds but one being slower than the other; because shorter scripts require less work than longer scripts, the client requesting the slower read should be charged a smaller fee.
Having said that, for long form reads only, I also offer the option to for charges to be calculated on the final duration length. You can find details on the Long Form Reads and Audio Books page.