You guys are lucky now a days that you get GC in year or two, there use to be time when it use to take 2 yrs to get perm cleared .

Any way I talked to couple of lawyers some time back and they mention hanging with sponser for six month is safe bait . I know one guy Indian who quit day before his GC is approved and he is citizen now . So looks like there isn't hard and fast rule. It's just safe bait if your case went to crappy officer at later date.