Also, for how many time should I switch on the lights ?

Grow in containers or tubs that are overwintered indoors or as year-round houseplants. Container plants may be grown as shrubs or trained as ornamental standards. Plants generally grow best when overwintering period is short. Since plants will tolerate some frost, consider leaving containers outside in fall until temperatures dip below 20 F. and return containers to a protected outdoor as soon as temperatures permit in spring. Overwinter in a cool location (40s F.) with moderate light and very little water or as a houseplant in a bright sunny but cool room with reduced water.

Jacky, where do you live in France? You can grow oleanders outside in many areas. Failing that do you have a greenhouse or conservatory where it will get plenty of light but cooler and less dry air than in the house? 20c is ok but not the dry air that would go with it.