In the most recent edition of European Child and Adolescent Psychiatry, an international group of researchers published a meta-analysis of studies done on neurofeedback. It shows that the effects of neurofeedback training on youth with diagnoses of ADHD are sustained at least six months after training.  The authors found their results promising and called for more study of longer-term effects, as well as non-specific effects.  You can find this study here and an overview of the study here.

https://link.springer.com/article/10.1007/s00787-018-1121-4

https://www.brainclinics.com/meta-analysis-confirms-sustained-effects-of-neurofeedback-adhd

Although neurofeedback practitioners know anecdotally from contact with former clients that the benefits last years, there are few studies that look that far ahead.  One that did was conducted in Australia close to 15 years ago.  It found that children with a diagnosis of ADHD who received 40 sessions of neurofeedback not only did not lose the benefits of training a year later, they actually improved their focus.