a certain amount of time 的意思是 一定的时间。
如:
You allot a certain amount of time to the markets each day.
你每天分配一定的时间研究市场。
Metamorphosis into an animal for a certain amount of time.
快速变化为动物,持续一段时间。
The railroad gives free transportation for a certain amount of baggage.
铁路免费运送的行李是有一定数量的。
A screen saver starts after the computer is inactive for a certain amount of time.
当电脑在一段时间不活跃之后,屏幕保护程序将启动。