Goodness, how can I cover such a wide topic in a few words!
I think that there is obviously no easy general rule. Depths of many geophytes I have experienced vary with habitat, soil type etc.
Colchicum is one that I feel I can say with some certainty are often deep. Twice the depth of the corm would perhaps be the norm though I have found Colchicum macrophyllum at least 30 cm deep in deep soil conditions. Though even this must vary as in the Aradena gorge in SW Crete it grows profusely as a chasmophyte on the gorge walls. They also produce their new corm in the same sinker style as tulips.
I must at this point say that in the course of my work I was granted permits to collect material by the Greek government! I also have been on several trips with the Cyclamen Society where we collected small quantities of Cyclamen again with permits for the Cyclamen Society.
Cyclamen, these can be at very variable depths again mostly to do with habitat. On slopes or very rocky terrain they can be sitting on the surface especially where erosion has taken place.
Cyclamen graecum and those of the repandum group e.g. C. creticum can be found at least as deep as 30cm and sometimes more. In these situations many cyclamen produce "floral trunks" which I like to describe as underground bushes in the sense that they appear to be obviously stems and branches and they are very perennial. These can be extensive in scree conditions especially with C. graecum.
Crocus , my experience on Crete is that they are generally shallow, by this I mean around the depth of there corm around 2 cm. This was often the case with Crocus laevigatus. C. oreocreticus was deeper and occasionally several cm.
Muscari, from little experience, not very deep though obviously I have described how they can have contractile roots and so potentially they could be deeper.
Arum, they can get to quite a depth especially in good soil. They do, I'm sure, have contractile roots which will pull them down. Dracunculus are probably similar. D. canariensis also produces stolons. Biarum in my limited experience weren't particularly deep. Many geophytic Aroids produce their new tuber deeper than the old one.
Tulipa are known to get themselves down deeper. They do this with sinkers where the new bulb is produced on an extended growth and placed below the previous seasons bulb. Some species also produce stolons which as well as running sideways can sink lower.
Galanthus. These are obviously bulbs of good soil conditions. I have seen G. reginae-olgae many times in the Peloponnese where it grows in valleys most often near the water course. Only on one occasion did I see the bulbs exposed, probably due to erosion on a steep bank but the bulbs had a huge build up of old tunics which had not decayed and were like a thick corky protection to the exposed bulbs.
I think that this is a reasonable range of examples based on my knowledge generally. Others may have other experiences which they can relate.
