m0useanswered grow question a month ago I think the more accurate estimate would be this, from their webpage on the product, despite it being in the indoor guide for it, it will be the same for outdoor.
" harvest around 7-8 weeks after flipping the cannabis light cycle "
https://dutch-passion.com/en/cannabis-seeds/gmo-fast
Its all based on when the plant enters its flowering phase. and it will be 7-8 weeks after it flips.
External light sources can mess this up. so flood lights, street lamps, flashlights, motion detector lamps.
My outdoors had so much light pollution I can only grow autos outside or they all hermi and don't grow well.
Fast just mean it can finish a week or two earlier then regular not fast plants. Its a gimmick and not worth buying IMO.
Just grow autos at that point, they finish early regarldess of lightcycle.