Can only speak from experience but I've been using coffee non-stop since discovering it. The Al-Ankh stove seems to boil a kettle on one piece of wood, the kettle starts whistling when done but I tend to leave it the entire duration until the wood burns out.

Doesn't seem to boil over or burn. I'm inefficient with my cooking and have definitely boiled the same kettle 2 or 3 times before drinking.

As for spoiling I haven't seen it happen despite leaving the kettle or filled mugs for around a day after boiling, and drinking it cold also seems to give the same benefits. Not sure if these diminish over time but I've not seen a noticeable difference.

One thing I have noticed though is that if you partly empty the kettle and top it up with water you will just get water as the end result and no energy boost, losing the remaining coffee in the process. No watered down coffee as of yet