In the Literature accessable to me I do not finde exact dates given for when it is believed he wrote this piece.
I would be greatfull for every answer that can narrow it down to a spann of less than 5 years. Sources welcomed.
Drobner (The Fathers of the Church, 182) gives it as after 314/315, while he was in Trier. This is also after the end of the persecutions. That gives you a start date. It has to be before circa 325, since Lactantius probably died in that year.
If you want to get into this, then the bibliography here is a good place to start. If you have trouble/need help obtaining specific articles, let me know via DM.