There have been multiple follow-up studies to answer this question. Follow-up research has been done on neurofeedback for ADHD, autism spectrum disorders, and PTSD. The period of follow-up has ranged from six months to three years. All of them show that the gains from neurofeedback last over the period of follow-up. Several studies showed continued improvement after the training ended. However, much more follow-up research needs to be done, and over longer periods of time.
Please remember though that these results are averages. If on average the gains last, still there may be individuals within the group whose gains do not last. In practice, we have found that for some individuals, the benefits from neurofeedback have lasted. Others need occasional booster sessions, say three or four times a year. Some though have required ongoing training at home to maintain their improvement. This is usually with more severe and more chronic problems.