Audio problem: After the Cyclops finishes drinking wine, he throws the bowl back at Odysseus. In that shot you can hear the sound of it hitting him and falling to the ground. But the bowl doesn't actually hit him until the next shot.
Continuity mistake: When Athena says, "Look at them, I fear for you," Polites is laughing at a joke the sailors told him, but in the closeup Polites is eating food out of a bowl.
Continuity mistake: At the end of the Charybdis scene Odysseus surfaces from the water and grabs onto the mast of the ship with one hand, but in the next shot he's grabbing it with both hands.
Continuity mistake: When the men are leaving the Trojan horse, there is a spear leaning at the center of one of the wheels, but when Odysseus starts fighting, the spear is at the far side of the wheel.
Deliberate mistake: When Penelope gives birth to Telemachus it is obviously not a newborn baby.
Continuity mistake: The cyclops throws the bowl at Odysseus and it lands on the ground, but it is gone a few shots later as one of the men says "I will kill him now".
Continuity mistake: When the men are about to stab the cyclops in the eye, Odysseus' hands are near the top of the stick. In the next shot they're further down the stick.
Audio problem: After Hector has challenged Achilles, his lips keep moving but no sound is heard.
Continuity mistake: After Penelope says "I am alone now", her head rotates between shots.
Revealing mistake: It's stated that Achilles has been killed. He then is shown lying face up on his pyre. But if you look closely enough (zooming in might help) you can see that he's still breathing.
Audio problem: Having just captured the pig, the soldier with a knife says "I'll stop your squealing" but his lips don't move.
Continuity mistake: Odysseus' hair constantly changes throughout the scene where he is talking to Athena on his ship.
Continuity mistake: Aeolus changes position on his chair between shots of him saying "Everyone knows Odysseus, the great hero who built the Trojan horse".
Audio problem: At the very beginning of the film, when Odysseus is running to his wife at Ithaca, we hear him breathing rapidly. His breaths does not match his mouth.
Continuity mistake: In the scene where Odysseus is climbing up the hill and sees the god Hermes, Hermes plucks a plant to feed Odysseus. Odysseus' head rotates to the left between shots.
Continuity mistake: Telemachus drives Antinous to the wall with a spear and stakes him to the wall. The spear is actually gone when Odysseus looks around the room after killing everyone.
Continuity mistake: When Antinoos throws the bow to the ground after failing to string it, it lands tilting upwards. But by the time Odysseus picks it up, the bow is lying flat on the ground.
Continuity mistake: Throughout the battle in the throne room at the end, the position of many of the dead suitors changes between shots.
Continuity mistake: During the Trojan war, when Odysseus and Achilles get surrounded by Trojan soldiers, the position of the soldiers change between shots.
Continuity mistake: When Penelope is making olive oil she pours a basket of olives into another empty basket. Between shots, the basket with olives moves closer to the empty basket.
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.