I suppose the answer is whether by paganism you mean the organized, urbanized cults of the Greco-Roman world or the polytheistic religions of the non-Roman world.
Dating the decline of the former is a bit tricky: paganism certainly outlived Constantine I (although he did create certain barriers to pagan institutions); it was badly damaged under Constantius II, who pursued a more active program of anti-pagan legislation; Julian the Apostate lead to a brief restoration andf then relative toleration under Jovian, Valentinian I, and Valens; then Gratian and Theodosius started the anti-paganism machine up again; then there was a revival of paganism after the death of Theodosius, amd so on. Some scholars use 529 CE as the end-date, based on the closing of the Academy of Athens.
The latter persisted a lot longer. Charlesmagne was fighting Saxon pagans in the 8th century; when the Danes conquered northeastern England and established the Danelaw in the 9th century, with the last pagan king of Northumbria in the mid-10th century; Scandinavia took a long time to be Christianized and you see archaelogical evidence of paganism into the 13th century; Lithuana wasn’t Christianized until the 14th century; etc.
Generally, the further east and north from the Mediterranean you go, the longer paganism held out.