This is a great question and most people have this concern.
The answer for how long will it take for a treatment to be effective is it depends on each individual. Some people response right away in the section of treatment, some may take couple visits. If you have been an acupuncturist and get treatment more than 6 visits and there are no signs of any changes at all. I will suggest to seek for another acupuncturist. It simple means this practitioner is mot best to suit your need.
Will it be ongoing? The answer for this is it should not. when you restore your energy and take good care of it. You no longer need acupuncture treatments.
Some people keep treatment for maintenance. To keep their energy is good way. if you don't want to continue during this period of time. It is still okay.
Hope i answer your questions.
Have a great one
Usually we suggest at least 5-10 sessions for resolving the problem completely (speaking of healthy individual overall), however the amount of treatmenys varies on case by case situation ( depending on underlining conditions in other words, what causes the migrane in each paticular case) Sometime single treatment could resolve problem. It doesn't have to be ongoing as long as your acupuncturist knows what he/she is doing.
Generally, for pain/headache, I tell folks they're looking at 5-7 treatments. After that, we generally start spacing things out until we hit some optimal interval that leaves the patient with sufficient relief. I've had patients with intervals as long as every 6 months and as short as monthly. The idea is that we find some space where the patient doesn't have to come weekly.
If the practitioner you're thinking of seeing offers a free consult, I highly recommend starting there. This will give you a chance to feel the person out and see if it's a good match. At some point, the practitioner should be asking about your expectations and giving you some idea of how previous patients with your issue have fared under their care.
Some people get immediate relief that lasts for months to the rest of their life. Some patients get relief for a few days, some for a few months, some for a few hours. For those that get the more constant returns, if they continue with the treatments it allows the body to continue balancing until the problems become less and less frequent.
Health and wellbeing is never a constant state - it is always a continuous journey. I hope you find relief and I hope you find a good practitioner that will guide you to a long journey of health.