Most of us have a love/hate relationship for tapering. We love to back off on our training and begin to rest, but we don't like the feeling that tapering gives us. It makes us feel lazy. Also, because we are running less, we feel we are burning less calories (which is true) but we all begin to feel we are gaining weight. It messes with are mental psyche, however it is a necessary evil that factors into our race performance on race day.
So how long out do you taper? I've done it all...3 weeks, 2 weeks, 1 week,....even the four day taper. What I have found is the two week taper works best for me. It gives my body the time needed to rest from the hard training but at the same time give it enough time to build back to up to peak on race day.
So, the next question is how much do I run during the tapering period? Well, I try to avoid the ever popular complete day of rest. I try to run almost everyday but I cut the mileage in half. I make sure each run I do has a purpose. For instance, if I am training for Western States, and I actually am, I would make sure to get out in the afternoon heat to simulate what I may run in weather wise on race day. Also, I like to mix in a couple speed work sessions to get those fast twitch muscles awake for race day. Nothing really long but enough to get the job done without fatiguing the body. The goal is to keep the focus and intensity of the workouts the same as you would each and every week.
Lastly, one of the tapering questions I'm always asked is, "Should I run the day before a race?" My Answer is yes. To be honest, I used to always take the day off before a race, but I always felt flat on race day. Moreover, I realized that the worst workout I would have each week of training was the run after a day off, and the best run during the week was the second run after a day off. So, I use the day before the race as an opportunity to get the "junk" out of the legs. I will run between 20 to 30 minutes with a couple of strides thrown in after I have warmed up.
If done correctly, tapering can help you perform better on race day.