The rule of thumb that I've heard for air drying hardwoods is 1 year per inch of thickness. That's assuming a fairly dry location like a garage or barn, elevated above the ground, and properly stacked and stickered. If you paint the ends of the boards with wax, or scrap paint, it may prevent some end checking. But unless the wood is quartersawn, some checking will likely occur anyway.
For spalting, the method I've read about involves putting the wood on shaded damp earth that's been covered with sawdust from spalted wood, and then cover with a tarp to keep moist. Check periodically to see if the fungus is doing it's work. There's supposedly a fine line between spalted and "rotted away"