Ele não está falando sobre a onda, ele está falando sobre o tempo que levará para drenar os motores da água e retornar ao espaço, observando que eles estão em um poço gravitacional tão grande que o tempo no planeta está passando muito mais rápido que no Endurance:
A novela explica isso um pouco melhor do que o roteiro;
“How bad?” he asked, thinking that he most likely didn't want to know. “Every hour we spend on that planet will be maybe…” She did the mental computations. “Seven years back on Earth.”
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Por acaso, no final, eles não demoram tanto a deixar a superfície como temiam, evidentemente devido à idéia inovadora de Cooper de soprar ar através dos motores para limpar a água.“How long to drain, Case?” he asked.
“Forty-five to an hour,” the robot informed him.
Cooper shook his head and uncoupled his helmet. The cabin was pressurized. Everything smelled wet, but it didn't smell like seawater or a pond. It smelled like distilled water that had been dumped on hot rocks—a mineral scent, but not salt.
“The stuff of life, huh?” he said. “What’s this gonna cost us, Brand?”
“A lot,” she said. “Decades.” Her voice was flat.
Cooper felt like he couldn't breathe. Decades. Tom and Murph were adults already. How old? It seemed impossible. He rubbed his face, trying to comprehend it. He watched the wave go, knowing there would be another, and soon.