Q: time flexibility for US immigration Post_602010-07-08 23:19:44

I am sponsoring my sisiter's family to immigrate to US in FB-4. It is getting close to their priority date. In this process, is there any time flexibility for convenience purpose? At what points and how to control the timing?
It seems that the time to return Choice of Agent is one control point for time flexibility for up to one year. Any other points?
How long is the entry visa valid typically?

THANKS in advance.

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