拿到绿卡后如很快被解雇,会被收回绿卡吗?

I have a question regarding to the following paragraph.

"However, the foreign employee has to work for a reasonable period of time for the sponsoring employer after s/he received permanent residency." (http://www.hooyou.com/lc/index.html)

How long is the "reasonable period of time"? is it clearly defined? some people say 6 months, is it true?If I got laidoff or leave the company soon after I get the green card, will it be a problem? Can I still keep the green card? Thanks.