3" is about the maximum thickness that can be easily, cheaply, and reliably dried. Above 3" and the costs skyrocket, plus you'll get dramatically increasing problems like checking as corners get cut with the drying/kilning.
There's a lot to be said for buying really thick blanks direct from local tree surgeons, doing some rough initial turning to dramatically speed up the air drying process, then finishing them off after a year or two in a plastic bag with a handful of shavings.