st, yes, usually a 30 minute rate applies to the first 30 minutes only . If you book a 30 minute rate, you are expected to be done/out within the 30 minutes.
Second, as an overview, you will never figure out provider rates. Don't even try. There is often no rhyme or reason as to a rate structure.
A lady could charge $500 or 30 minutes (because she hates 30 minutes, but will do them at a high premium) , $600 for the hour and only $700 for 90 minutes, because 90 minutes is her preferred length. The next lady charges $500 for 30, $1000 for 60 and $1500 for 90 minutes. Straight math.
I once saw a lady who charged $400 for an hour, $500 for two hours and $800 for an evening date package. Well, our evening date lasted close to 6 hours. She absolutely loved longer sessions (this included dinner out/time in) and that was her preferred way to see clients. So it was an enticement.
So never guess on rates, because often times you can't figure it out. If your desired length of time is not listed within her ad/website, just ask.