re: Green bowl newbie questions…

Unless you want it to be totally dry when your turn it, there is no min or max wait time.  Large blanks will take a long time to air dry -- years for large ones.  Even if you wait a year or 2 for a large blank before you turn it, it will likely still be too wet to finish turning at that point.  What I usually do with green wood, whether I turn it immediately or a year or two later, is rough turn it to a thickness of about 10% the diameter of the rough bowl.  It is mostly the rim that needs to be that thickness so you can actually make the rest of the bowl closer to the final thickness but leave a little room for final shaping.  I will then scoop up some of the wet shavings and store the rough bowl in the shavings in a paper bag or cardboard box to let it dry more evenly.   Fold over the bag  or put a lid on the box to seal it.   I weigh the rough bowl and write the date, weight and species on the bowl before storing it away.  I will take it out periodically and weight it to see how much weight it has lost and write the new date and weight on the bowl.  When it stops losing significant weight, it is ready to finish turning.  It may take a few weeks up to 6 months before it dry enough.  BTW, it is a good idea  to sort of fluff up the shavings every week or so until they do not feel wet anymore to prevent mildew.  When it is ready to turn, it may be warped so if you used a chuck, the first thing you will need to do is mount it between centers and reshape the tenon or recess so make sure that you leave a little room to make the tenon a little smaller and still fit in the jaws of your chuck.  You can just a use brown paper grocery bags but I use large paper yard waste bags and store multiple bowls in the bag.  

When I do not want to wait for the rough bowl to dry on its own, I will use a microwave to speed up the drying process.  It will vary based upon size, species and how wet it is at the time but I generally start by weighing the bowl and microwave it for 4 or 5 minutes on 50% power.  Remove it from the microwave and let it cool.  Reweigh and repeat.  As it starts getting drier, reduce the power to 30% and shorten the time to avoid charring the wood. When it stops losing weight, it is ready to turn.  I can usually dry a rough bowl in  4 to 24 hours depending upon how large and how wet it is and how long I wait between the next cycle in the microwave.