But to answer your question since you're so anxious to know, I doubt it's 6-8 months.. I think it'll be longer. They always prolong it.
KIt used to be every three months, but no one knows when the next update could be, it might be next month (highly unlikely) or next year as in sometime Feb/March 2008
Who knows? The last update was six months, but there were "pre-updates" and smaller updates since, to penalize directories and link sellers. Google looks as if they are changing their business model (again) therefore coming up with a date is anyone's guess.
Is that after Google has taken control of Earth, and have decided on 32 day months of the year, as it means more days in the year for them to sell Ads on Adwords ??
Only Google might be knowing it. One says for every 3-4 months, but google is always mysterious to predict.