三天打鱼，两天晒网 Sān tiān dǎ yú，liǎng tiān shài wǎng
This literally means 'three days fishing, two days drying nets', but the expression is used to indicate a person who cannot stick to anything.
The nearest English equivalents are:
'work by fits and starts' / 'lack perseverance' / 'play fast and loose' / 'blow hot and cold'
For example 学习汉语不可以 “三天打鱼， 两天晒网” Xuéxí hànyǔ bù kěyǐ 'sān tiān dǎ yú，liǎng tiān shài wǎng'. 'When learning Chinese, you cannot play fast and loose.'