Northern Lights and crap are never in the same sentence, NL is one of the best and most stable strains ever. Give it the love it needs and take it to ripe, and you will be rewarded with some awesome smoke that will kick your ass. The NL I used to grow would ripen in around 55-60 days, and it was very rewarding, both in yield and powerful smoke.
But if you must take it early, than 45-50 days is the minimum I would take it, you would lose some yield from the early cut, but you will still get some bomb smoke.
Just curious as to why you don't think they will give you a great turnout? Are they in bad health?
Well, like I said, these NL's are CRAP! Not sure what their problem is, but I have white widows and road runners that I have raised the same way, and the are absolutely stunning. My only guess is the company I purchased them from. These girls kept growing taller and taller and not out at all. There are what are like popcorn buds here and there, but barely BARELY any trichs at all anywhere. They have been in flower for over 2 months now. They look like ichabod crane! lol I have tried everything to find out what is wrong with them, even posting on here to get help and opinions, and they are just yucky. If it wasnt for the 50 or so little nugs on all 3 plants, I would have just thrown them out weeks ago. But I just keep hoping they will do something.
If a week of more light would make a difference, I will leave them, if not it is a waste all around anyway. I am thinking even a week longer would be a week of wasted light. They have absolutely no large fan leaves left on them, they are under 600 hps with a chit load of added bottom and side lights,
I have 5 more seeds from the same company, and I am truly scared to give them another try. If every one of the seeds turned out this way that I already planted from the same pack, I am thinking the rest might turn out this way. Maybe they are not NL's at all, maybe they sent me bad seeds, maybe they are maple tree seeds, I have no idea, but with 24 hours of care to these brauds a day, and cant show no love back, i wanna tear them down with a machete.
So............Maybe I should also mention the little buds have not changed in about 2 weeks, no more, no fuller, no prettier, just uglier.