Not touring that time of the year thanks to no winter camping equipment. I ride year round as I don't own a car. For the winter months...

Down to 10-15F, depending on how strong the wind is(10-12 mph) down to 10 degrees, much above that wind speed than around 15 degrees I'll add more:
I wear merino wool socks with regular cycling shoes and neoprene booties. My feet get cool to slightly cold on occasion but as long as they are dry I don't mind and I just ignore it. Never had any kind of problems otherwise.
Cycling shorts and wind pants. I used Velcro around the ankles to keep the wind pants from getting caught on frame pump/water bottle holder/chain.
Long sleeve cotton t-shirt and wind jacket(nylon shell for both the pants and jacket, no lining) I typically keep the wind jacket unzipped(read why below).
Pop top wool mittens.
Sweat band and a skiers ear band.
Same helmet I wear during the summer, plenty of ventilation holes.

Below 10 or on real windy days below 15 degrees:
Cycling tights.
Add a fleece up top.
Sometimes I add a neoprene balaclava depending on how windy/cold it is.

This pretty much covers 99% of the winter here in New Hampshire. I'll ride 60-100 miles in a day dressed like this. I'm comfortable all day long and I can stop and be outside for 15-20 minutes without getting cold...HOW, do you ask. Let's look at the science of how the body operates.

First, the body is designed to keep itself at 98.6 degrees F. Below that you start to shiver to warm up or make other movements with the body to warm up. Above that is where the key comes into play. Above 98.6 degrees you start sweating. Sweat is designed to cool the body down, hence the other name for sweat...evaporative cooling. When you start sweating your body is trying to cool itself down, it's not trying to keep itself warm. So the question comes up, why do you want to sweat if you want to stay warm? Pretty stupid isn't it.

Now when you stop, whether it might be to fix a blown tire, a broken chain link, etc., you have all this sweat on your lovely clothing, even smartwool clothing. This sweat is continuously sucking the heat right out of your body. The heat will continue to leave your body as long you have sweat on/next to your body. The manufactures of smartwool and other wool products want to make you think their product is so great because it wicks the sweat away. The problem is as long as you are sweating you are also wicking the heat right away from the body, at a much faster rate than you would be if you remained dry. The secret is to remain dry in the first place. HOW???

By wearing my limited clothing during the winter and keeping the wind jacket unzipped I keep myself warm by keeping myself dry. I let the ventilation keep the area between the shoulder blades(right behind the heart) dry. By keep the head uncovered I let the heat escape and not trap it in like a fool does. This keeps the heat from building up to the point where I start sweating. By not sweating I stay warm because I'm not suck all the body heat right out of myself.

I pretty much never remove any clothing when I ride. If you have to remove clothing when you ride you should have left the clothing at home in the first place. The first 20 minutes or so of the ride you want to be cold. If your not cold right off the start, your day is already over. Your done for and your clothing is going to be wet in no time at all. The wet clothing is going to be sucking the heat right out of your body...FAST. It's not about removing the clothing it's about avoiding sweating in the first place. You avoid the sweating by not putting the clothing on and by giving yourself plenty of ventilation so the heat can escape and not build up to the point of making you sweat.

Now one other real BIG secret is to start riding cold right now. As the temperatures start to drop, keep the clothing off. I was out last night until 9:45PM riding. It was 50 degrees and I had on cycling short and a long sleeve cotton t-shirt. I ride cycling shorts down into the 30s with nothing else on the legs. Let your body get use to riding in the cold. Let you body get used to starting out cold and let the exercise warm you up...not the clothing. The same way RAAM racers head to the desert and the high peaks of Colorado before the start RAAM to acclimate to altitude and heat, you want to do the same thing to acclimate to cold weather riding. The more you let your body get used to riding in the cold the more it adapts to riding in the cold. Don't start putting on the fleece and wool junk now when its 40-60 degrees outside. Leave that stuff at home for when it actually does get cold. Let the body adapt and you'll find you don't need that stuff more than a couple days a winter.

Learn to listen to common sense and not marketing pitches. When you see something in an ad ask yourself what are they trying to sell me and then ask yourself does this make sense. When it comes to the STUPIDwool and all this other crap they market the products to you by fear. Look at the way they market the products. Remember...if they don't sell the products they go out of business...that's how important marketing is to any company. Look at the flashy girls on the car hoods that the car companies use all the time to try to get you to buy their automobiles. Do you need the car...of course not. Do you want the car...no you want the girl. The marketing works. They know how to sell fear to you and it works. Sweat kills simply because it sucks the heat right out of your body. It's not the cold that kills, it's the sweat that kills(at least in the case of cycling). Keep yourself dry in the first place by keeping the clothing off and you'll stay warm. Add the clothing and sweat your butt off and you'll freeze your butt off as well.

Now I'll digress and agree with what others have said. Everyone is different. You do have to find out how little you can safely go with. Don't fret over adding more clothing, just fret over taking clothing off. Take the clothing with you in case you need it. Ask yourself the most important question though as you are riding...Can I keep riding like this all day and still be 'comfortable'. Notice I didn't say toasty warm, like sitting by the fireplace at night, I said 'comfortable'. Again, if conditions remain the way they are right now, can you keep riding like this all day long and still feel 'comfortable'.