一口吃不成胖子
yì​kǒu​chī​bù​chéng​pàng​zi

Rome wasn't built in a day

一口吃不成胖子 yì​kǒu​chī​bù​chéng​pàng​zi
Rome wasn't built in a day
literally "one cannot get fat with a single mouthful"; used as a proverb to mean that nothing significant can be accomplished in a single effort or that any great achievement requires time and persistence