I've referred to George Sheehan before. He was a cardiologist and marathoner who wrote for Runners World. He used to say "We are each an experiment of one." The point being, we each have to learn to listen to our own body to learn what we can and can't, should or shouldn't do.The problem is that there's no real quantifiable way (that I'm aware of) to determine if you're fully recovered or not, so we just end up going on feeling alone. Am I the type that will recover in 4 days? Or the one that might need 10 days? Who knows? The only tangible sign that we haven't fully recovered yet is muscle soreness but that doesn't always happen with every workout, so I don't think that going on muscle soreness, or lack thereof, is a very reliable indicator either.