Originally Posted by
Momscar
The short answer is as many as you can. Realistically, Dairy, sugar, bread oven, pie oven, cake oven and the grill. I have the feed mills maxed out because so that I always have feed available on the ground. The loom, too, so that you can make sweaters and hats while you are away.
I would say that if you fill your queue and it finishes shortly (an hour or so) before or after you anticipate being back on your farm, then you probably have enough slots.
Also, when you can, I would suggest acquiring as many of the smelters as you can afford. The achievements that go along with the mine/smelters take quite a while, and some of them are timed. So if you have 3 smelters running 24 hours, you actually get credit for 72 hours. And you will be able to work off any ore that you mine quicker.
If you are going to try incorporating any of the tips, start slow so that you can get used to a different way of playing. If you jump right in you may get frustrated and go back to your old ways.
Enjoy, keep the questions coming. :)