If you're having a bad session, and just not "clicking", then a 1/2 hour can seem too long a time to spend with someone.
If you're having a great session, then a 1/2 hour can seem too little.
I've had both.
When I had more money, I generally did 2 hour dates (even with non-porn stars) so I had time to talk with them, and relax, and enjoy myself many times during the 2 hours. Hell, I remember going up to Boston where there was a great agency that was doing 4 hour specials for $700 which was well within my budget at the time.
As others have said, sometimes the escort will give extra time if she is enjoying herself. Sometimes that's an extra 15 minutes, sometimes it's a half-hour, and sometimes it's longer. I remember an escort in Australia that I booked for an hour stayed the night and refused any additional payment.
The thing is, I treat all of these "sessions" as "dates". And just like any date, sometimes you hit it off, and both want more together time, and sometimes you don't. Sometimes you hit on a woman for whom this is truly a business and she's got a timer on her phone to tell her when your hour is up. Every "session" is different and as far as I'm concerned, trying to find any hard and fast rule can be self-defeating. If you go into a half-hour session knowing you only have those 15-20 minutes of playtime, you can be rushed and then neither of you will truly enjoy the session. On the other hand, if you're just there to meet the star, get your rocks off, and get an "introduction" that can extend to longer time in the future, it can be worth it if you "click" with her in that 15-20 minute period.
But, for me, personally, I think 1-hour sessions are the minimum I'd do with a porn star. I want that time to chat with her, get to know her outside of the film frame, and treat her like a person rather than a piece of meat that I'm there to use for a few minutes and then leave. I think I have better times with them treating them like a real person rather than a toy to use and discard