In Goblet of Fire, chapter 35, Veritaserum, while being interrogated, Barty Crouch Jr describes being smuggled out of Azkaban Polyjuiced as his mother. He goes on to tell Dumbledore that Barty Crouch Sr held him captive under the Imperius Curse and an Invisibility Cloak, with Winky the house-elf keeping watch over him. Sirius says that Barty Crouch Jr died about a year after being brought to Azkaban, but I can't find any information stating when that was. He was held captive by his father for many years, until Voldemort came for him the summer before Harry's fourth year and released him from the Imperius Curse.
For how many years was Barty Crouch Jr held under the Imperius Curse? I'm looking for a canon-based answer. Please no answers or information from the HP Wikia.