A long time away. As far as I'm aware it's still a fair way from release standard and I don't believe Mark's currently working on it - he has other projects up his sleeve at the forefront of his mind for the moment..
Currently, only Mark is using it. Early pre-releases were really incompatible and buggy when mixed and matched with different versions of Vanilla. When Vanilla is rock-solid stable and (it's almost there) I'm sure we'll hear more about it, but until that time I wouldn't hold my breath. I don't think Mark's made any conscious plans to "Duke" Swell, but he'll let us all know the status when he gets around to it :)
But yeah, he's a busy man and has to make a living so this is really secondary.