I'm a cat sitter, so thought I'd put my thoughts in.
With your username I guess you booked through cat in a flat, which is the website I use too.
If your cat sitter is stating that they'll be doing 30 min visits then that's what they should do unless you've agreed otherwise. On rare occasions I've had to cut a visit short and I always let the client know, and if it's a 2 visit a day client, generally make up the time on my other visit.
I am so paranoid about not short changing my clients, that if they have a ring doorbell or camera or something I take a photo as soon as I arrive to make sure I remember the time and don't leave a few minutes early or anything.
To those talking about travel time etc, then no, that's not to be included as standard with cat in a flat, if that's included then it should be discussed as part of the booking. You can really only advertise and book within a certain radius of your address on the site, so it's expected that you are willing to travel to cover the radius you set.
OP - please either bring this up with your sitter, with cat in a flat, or simply find a new sitter. You're not getting the service you're paying for and it gives the rest of us a bad name.