[Siegmund]

Rereading jason's solution, this is making a bit more sense: jason has started his frog at 0, and asked how many points in (-L,L) get visited as L gets large. The vast majority of points left of the frog's starting point never get visited, since the frog drifts to the right.

That is, the fraction of points in (-L,0) that get visited approaches 0 as L becomes large, trivially; while the fraction of points in (0,L) approaches ~0.854 as L becomes large (and this is the one that takes all the work.)

Yes, the original post was a bit ill-defined.