When asked how long version 1.0 of Stack Overflow would take to produce, Jeff Atwood famously replied "six to eight weeks" without any sort of serious planning or consideration. It took quite a bit longer. Listeners of the Stack Exchange podcast got to hear Joel Spolsky make fun of Jeff for this many times.
And since then, nearly every major upgrade, feature request, etc. has been scheduled as "6 to 8 weeks" or occasionally some variant like "6 to 8 months"
After Stack Overflow, we should really start referring to this as an "Atwood period".