There are two basic types of lily seed germination: epigeal and hypogeal. Epigeal is usually rapid, and the cotyledons rise above ground, followed by true leaves. In hypogeal germination, the seed first produces a tiny bulb and nothing above ground the first season. A cold treatment is needed before leaves will grow above ground.
These two forms of germination are subdivided into immediate and delayed germination. Most epigeal(E) germinators are immediate(I). Most hypogeal(H) germinating species are delayed(D).
chalcidonicum E, often D
humboltii DH, sometimes I
speciosum I or DH
Information taken from Lilies, by Edward Austin McRae.
Andrew, to begin with, I am assuming you know if it is a warm germintor or a cool germinator(American species), and are working with that. See here, one of the good sites: http://www.lilyseeds.com/growing.html
In fact, right now I have both warm and cool germinating delayed hypogeal seeds in fine vermiculite that have germinated and produced their tiny bulb. I haven't encountered any problems with the vermiculite.
Funny you should ask about how long it should take. I also had L. auratum and L. martagon in vermiculite upstairs for their warm period (68 F). Some of the seed was along the bag edge, where I could monitor them. They almost immediately plumped up, and did nothing. Then it looked as if they were going to go, then nothing, and they went back to a more opaque look. Then after a while they turned more translucent again, where I could easily see the embryo inside. Success . . . well, no, and they returned to their plain old plump look. Something was literally preventing them from germinating. Kinda forgot about them for a while, until had what turned out to be a bright idea: I took the seed brought them downstairs, sat them on the floor along the outside wall. A thermometer placed there gaged it at 64-65F. Within a week auratum was off and running, and it took two weeks for martagon.
Oh Sherleft, thanks for researching and condensing all that information. I'm going to copy it to my thingy and print it out for reference. It's seems like with my wide variety of specie seeds I am always referring to Ed's book.