The aim of this systematic review was to investigate the effect of treatment duration on children’s behaviour and/or anxiety in the dental setting. For this purpose, a systematic search was conducted in Pubmed/medline and Scopus from 1970 to march 2017 for English language articles that assessed the relationship between dental treatment duration or length, and fear/anxiety or behaviour in children aged <12 years old with no confounding medical and/or psychological history and neuro-psychiatric disabilities. Four studies investigating the effect of treatment duration on children’s behaviour during and/or after treatment were included. None of the reviewed studies investigated the effect of treatment duration on children’s dental anxiety or fear. There was a general tendency towards deterioration of children’s behaviour with the increase of treatment duration. In conclusion, our results undermine the validity of current suggestions about the appropriate treatment duration. Further clinical trials are needed to establish appropriate treatment duration for more effective behavioural management of paediatric patients during dental procedures.