Continuity mistake: When Telemachus is in the dark trying to string his bow, a vase with arrows in it is visible next to him. Then after he throws his bow to the ground in frustration, the arrows are gone.
Continuity mistake: In the scene where Hermes is talking to Odysseus on the cliff, the nearby plants changes distinctly between shots.
Continuity mistake: There are two shots of Odysseus floating in the water right before he reaches Calypso's island. In the first shot, you can see that he is wearing armor. In the second, his chest is bare.
Continuity mistake: When Hermes says "Heed my advice and reach that stone" Odysseus is looking at Hermes. In the next shot, as Odysseus replies "Which?" he is facing up the cliff.
Continuity mistake: When Odysseus is hugging Calypso, the ship magically appears on the horizon.
Continuity mistake: Between shots of Menelaus saying "You, Telemachus, you must return", his arm magically appears on Telemachus' shoulder.
Continuity mistake: When the men are all holding onto the boat so as not to fall into the whirlpool Charybdis, Polites' face is covered in blood. But when he falls, most of the blood is gone.
Continuity mistake: In the scene where Odysseus is talking to Hermes as he holds onto the cliffside, the position of his arms changes between shots.
Continuity mistake: When Odysseus arrives back home and comes to Eumaues' house, the pitcher and cup on the table keep changing position.
Continuity mistake: Before Telemachus leaves Ithaca, he is shown having a conversation with Mentor. Throughout this scene Mentor's hands vary between clutching his staff and on his lap.
Continuity mistake: During the battle in the throne room, the arrows in Odysseus' quiver and the strap over the throne keep changing between shots.
Continuity mistake: The baby that Odysseus holds in front of Ithaca in the sunlight is different than the one Penelope has in her arms when she says goodbye to her husband.
Continuity mistake: When the cyclops first comes in the cave the air is really foggy, but when the men try to move the stone just a few minutes late it is pitch black outside.
Continuity mistake: When Odysseus sees Calypso's island and begins paddling to get there, the ropes on the mast go from being in front of him to being behind him.
Continuity mistake: When Odysseus raises the sail of the raft, it is obviously near sunset, but for the rest of this scene, it is much sunnier than it previously was.
Continuity mistake: As Melanthe and Eurymachus fall dead to the ground, the suitor speared to the door has his left arm raised. When Odysseus looks around the room, his hand is lowered, then after he tells Telemachus "Don't let your mother's eyes fall on this room" his hand is raised again.
Continuity mistake: During the storm at sea, Odysseus grabs onto the mast of the raft, and between shots his hands come closer together.
Continuity mistake: When the men enter the cyclops' cave and Elpenor falls to the ground, his position changes between shots.
Continuity mistake: After Prince Hector explodes out of his chariot, there is a shot of a soldier falling down and another one stabbing him with a spear. Then when Achilles starts yelling "Hector is mine. Mine." the same soldier falls down and gets stabbed again.
Continuity mistake: After Anticlea commits suicide, the grievous Euryclea turns her head toward the left. In the next shot, her head is turned toward the right.
Answer: The Trojan War itself was a decade long, followed by Odysseus' ten-year journey home to Ithaca, so all told he was gone roughly twenty years.