Usually dry to the touch overnight, ready for dulcote or another layer of oil. You can use a lampbox, lightbox whatever you've got to speed it up to a few hours if you're in a rush. Also, you can add Liquin and such to your paint to make it dry faster, as well as wicking off the oil before use by putting your paint on an absorbent palette.
Furthermore, if you really feel the need for speed you can get some of those heat set oils and a heatgun, although I've never tried it, I've read about others who have.
For me, I just use straight oil and let it dry overnight or longer. Military figure modelers and painters have been using oil paint for decades on their minis and there's a great long history of their use in the hobby and you can't go wrong with their approach and techniques. Every time I've had a question about oil on minis I've searched over at
http://www.hfmodeling.com/ and everything is discussed there in great detail on the topic.