The chronic unpredictable mild stress (CUMS) model of depression in mice is a model commonly used to investigate stress-induced depressive-like behaviours. The duration of the stress-inducing procedure is variable, thus making it difficult to compare results and draw general conclusions from different protocols. Here, we decided to investigate how the duration of the CUMS procedure affects behavioural changes, body weight as well as the level of plasma corticosterone in stressed and nonstressed C57BL/6J mice subjected to CUMS for 18 or 36 days. We found that 18 days of CUMS induced a robust decrease in grooming time in the splash test and a significant increase in the immobility time in the tail suspension test (TST) and the forced swim test (FST). All of these stress-induced depression-related behavioural effects diminished or even disappeared after 36 days of CUMS. Plasma corticosterone levels were increased in the CUMS mice compared to those in the nonstressed mice. However, this effect was more pronounced in mice stressed for 18 days. On the other hand, a gradual decline in weight loss in the stressed animals was observed as the duration of the CUMS procedure increased. Altogether, the results indicate that 18 days of CUMS did not affect body weight but caused significant behavioural effects as well as a robust increase in corticosterone levels, while 36 days of CUMS induced significant reduction in weight gain but only slight or even non-significant behavioural effects. These results may indicate the presence of adaptive changes to the long-term CUMS procedure in C57BL/6J mice.