People talk about 70 days...80 days...90 days...and no mention of what lights they are using...but it's ALL about the lights. I have Vero Cob lights and I have a few Platinum P300's, Vipraspectras, etc,..I like to experiment. I can harvest an auto White Widow in 70 days if I use my Cob set up...but my P300s are 80 to 85 for the same grow. Same with feed charts that come with nutrients...I can do close to full dose with good lights and about 1/2 dose with cheaper lights. it all comes down to ho much photosynthesis is going on.
A 70 day grow is possible, but only with really good lights.