How long firewood will take to dry will depend on the temperature and relative humidity where you live. A poster from Texas mentioned that trees dead (or perhaps logs down) for 2--3 years would be dry, but that is not the case in the North Carolina mountains; I've seen logs down 5 or more years that were almost as wet as green when split.
Burning a mixture of green and dry wood will give a longer burn, but there are two disadvantages. First, a lot of the heat from the dry wood will be used in boiling the water in the green wood so that it can burn, reducing the stove temperature, and less of the dry wood's heat is available for heating your house. Second, this increases the chance of creosote buildup. But you can build up crosote even burning all dry wood if you keep the airflow reduced so much that the stove temperature is too low. Ideally, a fire with more airflow and less wood produces less crosote and a higher temperature than minimal airflow and lots of wood; of course, the stove full of wood will go without refilling longer.
The ariticle on covering wood stacks with clear polyethylene was interesting. If you leave a 1-2 inch gap at the bottom and a small opening at the top, drying is quicker because you get a chimney effect during the day, with cool drier air coming in at the bottom and hot moist air leaving at the top.
