一口吃不成个胖子 Yì kǒu chī bù chéng ge pàngzi
Literally "one cannot put on weight by taking a mouthful of food".
This has the connotation of "Rome was not built in one day / cannot be reached in a single leap/at one stroke / cannot reach the goal in one step / cannot accomplish in one move". Chinese people use this saying when comforting someone who is losing patience in studying.
Example: 学习汉语不可以急于求成，请慢慢来，一口吃不成个胖子。 Xuéxì hànyǔ bù kěyǐ jíyúqiúchéng, qǐng mànman lái, yì kǒu bù chéng ge pàngzi 'One has to be persistent in learning Chinese, it takes time, just as Rome was not built in one day.'