The best time to trek the Bandiagara Escarpment is October to February, when the harmattan has cleared (or not yet arrived in force) and temperatures are manageable. The hottest months — March through May — can be brutal on the exposed plateau, with temperatures reaching 40°C or above. The rainy season (June to September) makes some paths difficult and may interrupt transport.
Note that the security situation in Mali has deteriorated significantly since 2012, and travel to the Dogon Country has been affected by armed group activity in parts of the Sahel. Check your government's current travel advisories before planning a visit. The situation changes; some periods have been safer than others.