Depends on the lines, pedigree, and individual dog. I've seen very nice pick pups turn out surprisingly bad and sometimes average pups not kept back turn out quite nice. But generally 7-8w will give a basic idea of how the pup is - ignore structure from 4m to one year - adolescents are very ugly and gangly as they grow. At about a year old, you can get a general idea of temperament, structure, and eventual development. Little later for slow maturing lines.
five day window at eight weeks to predict future conformation?
exactly eight weeks not a day sooner or later?
mom to the cashman
the rescues...jeni-take-a-ride, and the c-monster
shangri la's seraphina blue (RIP 8/1/15)
hearthside's cinderfella (RIP 4/20/09)
shep von bellefontaine (RIP 6/9/10)
cricket (african grey RIP 7/13)
voodoo lily (dsh RIP 8/15)
to accurately guess what a pup will grow into, you have to know what to look for.
Puppy Puzzle by Pat Hastings is a good video that walks you through how to evaluate a litter. Choosing the best puppy is another good video.
Puppy Puzzle shows you what bone markers to look for that will help with predicting how the skeleton will change as the puppy grows.