Follow up treatments should take less time, maybe 30 minutes to an hour. It is best to ask the practitioner you're going to see, because they know their clinic patterns.
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For pain patients I'm typically able to get some reduction on an initial treatment. This reduction tends to last 24-48 hours and the pain will start to come back. Usually by the 3rd or 4th treatment, patients are able to go several days with either decreased pain or no pain.
Most studies show acupuncture has a dose dependency - meaning that, to a point, more treatment is going to yield better results. These studies show that, on average, around 8 treatments produces a pain lowering effect that can persist for roughly 12 months.
In other words, acupuncture is very unlikely to be a "one and done" type of treatment. It can and does happen, but it's unusual when it does. There are a lot of factors which can affect how quickly you might see results and how long those results will last. Those factors include: the specific nature of your issue, how long you've been experiencing the issue, whether there are lifestyle adjustments which can be made that will affect your issue and how well you make those lifestyle adaptations.