dwc flush question

if it's an auto then expect to harvest around the 70th day. I would sling out the nutrient solution and replace with just water around 10 days before harvest, so about day 60

have a look in my signature for lowryder#2 DWC as you may find it helpful. it's a DWC grow and an auto. it was under a 250w

 

 
You have to visually assess when the buds stop chunking up. If they have then starting the flush isn't missing out on bud

 

 
sorry mate, I didn't realise there were new comments on there. I have replied now
 

 

Knowing when to harvest can be tricky, and everyone's method of doing so is their own opinion.   To truly know the optimal time to harvest a particular strain, it takes experience growing harvesting and smoking it!    
 
Autos are a different kind of beast.  Even relatively stable genetics in autoflowering cannabis will show different phenotypical expressions.  This means that they may not all ripen at the same time as the last batch.  The only way to ensure they do, is to take clones of a given plant (which doesn't work with autoflowers)
 
That said, I would suggest that you take note of take note of your plant's pistils.  When you think they are ready, wait a week.  Then before you harvest, you might even wait another week. A better way of doing that is when you notice that about half to 3/4 of them are orange/brown I would suggest that you begin your flush then.   (note that a lot of people would harvest at this time.)  Flush for at least a week.  You might even go two.  This is largely a personal preference, and you'll know  what's best for you after a few more grows.  But try one week of flush after most hairs turn orangish brown, and you really can't go wrong.
 
I tend to be patient, and let my plants mature for longer than a lot of people do.  I generally harvest once my plants show a lot of yellow and brown, and dried up leaves have been picked off.   But that's my preference.  Everyone has their own way of doing it. I prefer for the plant to use up its own internal storage of resources (what causes the plant to look green) which means there's less work to be done post cure.  Some people prefer bright green weed.  

 

thing is with reading stuff on the internet you can never be sure whether the person just made it all up. I have smoked bud from plants that I would consider early and what I would consider late and there's not much in it really. the biggest difference I find is if its early you miss out on some weight. and in my experience on grow forums most people harvest in my opinion too early. mostly first time growers that aren't patient enough or going off what the seed vendor says. they give a number of days based on the earliest practical harvest time, this is usually a week or two before the peak time. the peak time being my opinion of course.

if you see lots of amber trichomes I wouldn't hang about, but if you still have clear ones hold back. anything else is pure opinion

 

I second what Gold Grower is saying.   I think most people harvest too early, in general.   I've had a couple plants tested, and the results do vary by harvest time, but the trade off is a higher CBD count.  It all depends on hat you're after.
 
I've been at this for a long time, probably longer than a lot of people on these forums have been alive. I can't tell the difference between 19.8% THC and 23.1% THC when I smoke or vape it.  I think a lot of people are too focused on the things that don't really matter.  Rather than focusing on potency, I think the effect can be controlled to some degree.  I've found that by pulling plants later, they tend to take on more traits of a sativa high.  Pulling early, I find they tend to take on traits of an indica high.  I love both, each have a place in my life.  Each have their merits. 
 
Like I said before, knowing when to harvest at the best time for YOU, and for the strain your growing is purely based on personal preference and experience.  We can only give some basic guidelines based on our experience.  It's probably good to try my method, try Golds' method, try pulling one early, too!
 
I recently pulled a killer Sativa.  It's MTG DreamWeaver.  I got the seeds straight from the breeder at Cannabis Cup, the day he debuted them.  It's a cross between Alpha Blue x Blue Dream and it's recommended to go for 10 weeks. A pheno based on another seed finished perfectly at 8, but had a weak branching system (branches collapsed under the mega bud structures.  One pheno wasn't anywhere near ready at 8 weeks. She was still going strong at 10 weeks.  Based on how she looked, I was going to let her go for 12.  But I wasn't going to run the entire flower section for two weeks with one plant in there, and was going on vacation the next day, so I pulled at 10 weeks, when she could have gone 12 based on my taste.  This was the first time growing this strain, and 10 weeks made a killer killer end product.  I was so impressed that I'm going to do a few runs of just this one phenotype so I can learn when to harvest her.  I'll pull some early, on time, and late, just to compare the results.   Perhaps I'll have them tested and I'll document the results in a grow journal for all to see. 
 
This is a great conversation to have.  I encourage you to take note of when you pull, and how it turns out.  Next time, try something different.  Don't worry about test numbers as much as what ends up being a final product that you enjoy the most!