Well, 25% moisture is OK, even 30% will burn if you are out of dry wood. It will boil the water out and then burn. 20% is better and the 'goal' for many of us (my goal anyway). 12% is about as low as I can get and that is what most of the 2x4 and 4x4 non-treated salvaged construction wood is that I burn. For every 1% of moisture lost, you gain about 1% of heat value. Going from 30% to 20% is usually pretty easy. Below 20% gets harder and longer, especially for dense wood species. The species makes a difference, as does split size, tight or loose stacking, sun and wind exposure, temperature and average humidity. And of course how dry it is kept; you need to keep it off the ground and out of the rain, but open to air flow. Large barns or covered areas with open sides are better. You can also accelerate the process using greenhouses or stacking it in sheltered patios and the like. It is also better to stack the wood loose or alternate splits 90 degrees you will get better air flow and faster drying. Smaller splits dry faster as well.