They're fully grown about 6-8 months.
Also a real show-quality nethie is supposed to have a dwarf gene which makes them the small shape and short ears, the gene is dominant, and a rabbit with a double dwarf gene (I believe its called peanut in English) rarely grows up sadly.. which means the show-rabbits usually have only one dwarf gene, but this also means that occasionally a kit is born without the dwarf gene.
These may be a bit larger and have longer ears than a "show quality" Nethie.
You very rarely find a show-quality rabbit in a pet store, there are breeders (unfortunately) who deliver the kits they don't think are suitable for show to a pet store and keeps the "good" ones for themselves. The other pet shop bunnies may come from backyard breeders or people with a random litter, who don't really think much of show quality at all. It doesn't have to mean the rabbit is mixbred, they just stray from the standard a little.