It wasn't three years. Oh no, the wait was much longer than that. Wikus had to wait three years, 6 months, 1 week, and 4 days.
He spent an incredible amount of time in the beginning fantasizing about Christopher's return, never doubting that he'd make good on his promise. Even when the three year mark was up, he still did not doubt. (Wikus, of all people, knew about the red tape that Christopher probably encountered on the return to his planet. For an intergalactic mission, there surely was a mountain.) To begin with, Christopher wouldn't abandon his people to Earth. Wikus had seen his reaction to the experiments, had witnessed his rage, and no, Christopher would never just leave his people to be cut up and studied indefinitely. But more than that, Wikus knew on some strange, unexplored level that Christopher was not the type to break his word. So, for the first year after his transformation, he spent much of his time making gifts for Tania and daydreaming of the day Christopher came back to save him: how the prawn shell would fall from his body, the way it would feel to walk from District 10 as a normal human being, the look on his wife's face when he opened their front door and walked into their home. He'd sweep her into his arms, kiss her gently, take her to bed and make love to her for hours. Perhaps he would bestow all of the flowers and trinkets he'd created for her as a grand and magnificent symbol of his love. Sometime after that first year, though, he stopped having that fantasy. Sometime after that first year, he started stockpiling cat food.
Wikus didn't even realize he was doing it at first. Of course, he had always kept some cat food backed up. His friends (when did he begin thinking of them as friends? He couldn't quite remember) were often stupid about it, always almost out before they went to trade for more, but not Wikus . He knew the system, he worked the system, he saved up. Soon there was a secret stash underneath the floor, and he rarely touched it. Soldiers would often raid the encampment, but they never found his food storage. Once they did find some of his gifts for Tania, but Wikus thought that was because he'd actually started leaving them in plain sight. Really, though, Wikus was way too smart for them, and the fact that they treated him as if he was some kind of sub-form of intelligence made it even easier to keep things hidden.
In the days before the ship returned, the air began to change, growing heavier. The other prawns felt it, too, and they all stood in huddles and talked about the return, ignoring the soldiers that tried to break them up. Would there be a war? How soon could they they all go home? Christopher would be bringing reinforcements, he and his friends were sure, and they might be expected to fight first, or maybe they would all just board the ship and go. The camp was split, some of them pointing out that their numbers were now huge, and would grow larger with the upcoming surge, and now they would have a chance of paying the human race back for all of their "kindnesses", while others were tired of death and wanted only to leave. Wikus leaned toward the latter side of the debate. He couldn't do that to the people on this planet, he told the other prawns, some of them he had once known, once cared about. He knew they were living on the other side of that wall somewhere, and even if they seemed so very far away right now, he certainly didn't want them to die.
The day Christopher finally returned, the prawns all came outside to wait for the shadow to drop down over the city. The humans' radar systems hadn't even registered his ship yet, Wikus had to laugh to himself as they all stared up at the sky, so there was much confusion about "what could this all mean?" If the camera crews ever happened to just ask one of them, the prawns would have happily explained that today was the day that it would all be over, and the humans could make of that whatever they wanted to.
Wikus, for his part, spent the time thinking about his upcoming reunion with Christopher and, hopefully, his son. He knew an argument was coming, because Christopher would tell him that Wikus was meant to be among the humans, or what might be left them if there was a war, so he needed to take the antidote and remain here on Earth. And Wikus would have to tell him of the way this prawn body felt natural, now, more natural than his previous body ever had. He'd show Christopher the cans of fantastic food he'd saved for the trip - Christopher would be very pleased about that, he was sure - and maybe he'd tell Christopher he had missed him, that he thought of him as a friend. As family, even. And if none of that worked, if Christopher was still intent on returning Wikus to his human form, he'd have to tell him about the eggs that he'd inseminated recently. Obviously there was no way he could do anything other than go with them, now that he had children on the way.
It didn't even scare him, he realized, the idea that the transformation was almost complete and there would be no going back. Someday he might not even remember being a human once upon a time at all. Maybe there would only be the memory of his current form, and Wikus found that strangely comforting. After all, he didn't think about his wife hardly ever anymore, even if he did dream of her sometimes. Dreamed of holding her with his clawed and muscular arms and making her see what he'd become. Who he now was.
They could see the ship now, blocking out the sun, and Wikus was ready to go.