Odysseus spends 20 years away from Ithaca.
His absence combines the ten-year Trojan War with the ten-year voyage home described in the Odyssey. He leaves Ithaca to fight at Troy, and after the war ends, storms, divine hostility, monsters, and his own decisions keep him from returning for another decade.
During that journey, Odysseus encounters the Cicones, Lotus-Eaters, Cyclops Polyphemus, Circe, the Sirens, Scylla and Charybdis, and the island of Calypso. Calypso keeps him on Ogygia for seven years, one of the longest single delays in the narrative.
The poem opens near the end of the journey, with Odysseus still detained by Calypso. Meanwhile, Penelope and Telemachus have endured roughly two decades of uncertainty in Ithaca. The ten-year voyage should not be substituted for the full absence: ten years refers to the journey after Troy, while twenty years includes both the war and the journey home.