No hard and fast rule. Depends on the cut and how you like to smoke. I like it drier, easier to pack, light and smoke and improves the flavors as well. Not to mention less steam, less bite. If it is ribbon and you squeeze a pinch together and it doesn't either crumble or release back immediately, it's too wet. I mostly smoke flake. If I bend it in half and it doesn't crack/break, too moist. I pre open my tins by a few days to a month or two. SG blends open at least a month before I start smoking them and still have to usually dry.
If you have to dry, the best method I've found is to wrap it in a napkin for an hour or two. Microwave if you must but that does seem to take away a bit of the flavor. While I haven't been much of a B&M out of the jar guy, that tobacco has generally been dry enough right out of the jar unless the tobacconist just dumped a fresh bag in. Learn to judge by your own touch/feel. I'd recommend erroring on the dry side. If it doesn't crumble to dust, it's likely good enough to be smoked. I know that sounds counter intuitive but just try it.