Can you be 'cured' in just one session?

I am not convinced that anyone who tells you that, 'you only need one session and your problems are cured' is giving you sound advice. If it takes some time to create behaviours, then does it not follow that it also may take some time to change those behaviours too? Hypnotherapy and NLP have a good reputation of being able to help people to change behaviours very quickly, but I doubt that even the best of therapists would want to claim it only takes one session.

Of course if those therapists charge a huge amount of money for just one session, then they may claim to have great results, and who knows, some people may even believe that. For most people though, they are realistic enough to know that behavioural changes come about less quickly than just one session. I would hope that no therapist with integrity would claim to help people overcome panic attacks in just one session. That would be most unfair to clients.

Therapists also need feedback as to how the previous session has gone, and so if you only go once, how does that therapist know if you have made enough changes that will continue?

Best Practice would be that you go for an initial session, experience hypnosis, and see what happens. Go to each session with feedback so that your therapist can evaluate how you are doing. How does any therapist know how quickly you will change? Some clients need three or four sessions, others may need more. A good therapist will be able to judge by what language you use during the following sessions, and how you are making changes in your behaviour.