When should I start charging my Lithium battery? The comments in this question say that I shouldn't wait until low battery charge level before I charge, because that reduces battery life (this is contrary to my understanding before that the faster we charge (without waiting for battery to discharge) the worse it will be for battery life). My question is, when I should generally start charging to avoid this effect? 20%? 

Interesting question: If we always limit the usage of the battery only 20% and above, won't we, in effect, reduces our battery capacity to 80%, because we will only use the battery in 20%-100% state?
As soon and as often as you can. Spending time at any extreme of charge (high or low) can damage lithium batteries. High charge isn't that bad, considering how quickly modern devices will drain away from that state. Spending time at lower charge levels will damage the internal structure of the battery though.

Draining to "0" on the device won't actually immediately kill the battery though -- the battery protection circuitry will prevent it from going down to critical levels. But the more time you spend at lower charges, the more cumulative damage you do and the more time you take away from the lifespan of the battery. Do the best you can to avoid this, and you'll likely have a battery that still gives you 70-80% of it's life by the time the device is considered obsolete.