Android - When should I start charging my Lithium battery?

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.

Lithium Ion batteries don't develope "memory" like Nickel Cadnuim batteries, so charging every day regardless of your discharge level will not affect the battery life. If my memory serves me right, because of the way that the batteries are recharged it is less strain on the battery if you don't let it get below 40% but I wouldn't worry too much about it. In that same respect, don't be hesitant to charge anytime you have the opportunity.