It will take about 3 weeks, and you will know because you will be drowning.
Rip-
I've been reading this forum all weekend, and haven't run across this on here (or anywhere else that I can see online). I'm starting SS as a fatass (34, 225 lbs, 35% BF) who also does triathlons (see where all cardio gets you?), all based off the quote of "a stronger athlete is a better athlete."
I'm sure there will be some sarcasm of "Why the hell would you put your body through the tri?", which I fully understand, but I enjoy it. The main goal is to have better swim/bike/run times. Lifting goals are to get to 2x BW for squat and DL, bench 1.5x BW, and press BW.
That said, here's the Q:
At what point will my body and the CNS see that lifting M/W/F on a linear progression and running/swimming on T/R/S is too much, and how will it let me know?
It will take about 3 weeks, and you will know because you will be drowning.
Don't structure your workouts like that. I train a fairly successful tri-athlete and started her off on the SS program about a year ago. She ran or biked every day during the week (often both) and swam on weekends ( I had nothing to do with her tri-training).
Twice a week training was more productive than three times per week. Even with that...expect some bad days, especially on the squat and you won't get workout to workout progression like a normal lifter. Eke out gains where you can but don't force the issue if it ain't there, and some days it won't be.
Training you people tests my patience.
You can't combine the two and do either well. Sorry.
What you do is figure out how long it will take you to get into shape for your July 1st start of season. For the sake of argument, Let's say it's six months - meaning you need to start on January 1st. That would mean you have three months (Oct, Nov, Dec) for strength training. For those three months, just do SS - nothing else. In my experience, SS all by itself is enough to maintain your CV capacity at a reasonable level. You'll be out of shape of course, but it won't be like starting from square one. Your train up after Jan 1 will go better as well since you'll be stronger. You could also take advantage of that three month period to clean up your diet - don't eat less, but better, mainly meat and vegetables.