关于by which和by which time,怎么用?

2. The average length of labor during which these women received their meperidine was under 12 hours.
4. Socially Awkward Situations During Which It Would Be Acceptable to Mess With Texas.
5. Fish evolve a longer lifespan by evolving a longer period during which they reproduce.
4. Have been fostering for five years, during which time they have fostered 20 children.
5. ... registry will occur on 9/24/2006 from 9 PM to 9:45 PM EDT, during which time you will be unable to register, renew or manage .
这些都是google上搜到的原版英文句子,可以看出一个规律,during which的中心语大部分是表示时间段的名字例如period等；而duiring which time的中心语常常是具体的时间如5 years等.
具体的原理我不记得了,希望这样的对比能对你有所启发.