Dispersal of the proverbially slow land snails over suitable habitats is an interesting topic. Whilst the ranges of some species, such as Zoogenetes harpa, naturally extend across several continents, others are restricted to smaller areas.
Yesterday, I read a short paper by Baur and Baur titled "Dispersal of the land snail Helicigona lapicida in an abandoned limestone quarry" (Malak. Abh. 24:135-139, 2006).
The authors released 300 marked adult snails of the said species in a limestone quarry and then monitored the dispersal of those snails over a 2-year period. Although the recovery rates of the marked snails were low, 16.7% after 1 year and 4.0% after 2 years, the results do provide a general estimate of how far and how quickly this species disperses.
Fig. 1 from Baur & Baur, 2006. Arrows indicate median displacements.
Median dispersal was only 1.7 m 5 months after release, but increased to about 6.4 m after 2 years. These results roughly agree with the measured dispersal rates of other species of land snails. However, smaller species in general tend to move less within a given period than do larger species.
If we assumed that the measured dispersal rate of H. lapicida was constant over a long period and there was a continuous limestone habitat, the approximate median dispersal of the snails would be 32 m after 10 years, 320 m after 100 years and 3200 m after 1000 years. You get the idea. Dispersing barely 3 km over a thousand years isn't much of a dispersal. Over geologically long-enough periods other factors, for example, the movements of the continents, also come into play. Of course, we also have to take into account assisted dispersal facilitated by winds, animals, etc.