I think WesT hit the nail on the head (at least for me anyway.)

"Building time will also depend on how tolerant your wife and family are of your absence."

I'm building the Mill Creek 13 by CLC. I've had the plans for a year, and have been goofing off at work reading this board since before then, so I have built it a thousand times in my head already.

I actually began building it about 6 weeks ago. The hardest part is finding time. I find that after putting in a full day at work, getting my 2 year old bathed and in bed, and then cleaning up after dinner, I'd often rather just plop down on the couch with my wife and watch TV. She likes my company too, after being home with a toddler all day. I try to go down to my basement shop at least twice a week for an hour or two in the evenings. The weekends are often out of the question for me.

At this rate, I have probably only put in 12 hours in these last 6 weeks. I am building from plans, and so far have scarfed the sheets of plywood, plotted the panels, cut them out, and planed down to the lines. There is a lot more to go, but I'm finally actually making progress other than in my head.

My point is that the only way to get it done is to start. If I had started back when I got the plans, I would easily be done by now. I've heard estimates from 30 hours to 50 hours to do a plywood kayak. There are 15 weeks between now and September. If you work 5 hours a week, you will easily be done by then, even if you are a slow worker.

Talk to your family. Work it into your schedule. Ignore the summer reruns on TV. Get started NOW. You will easily finish in time.

Hmm... I need to follow this advice too.
-Dan