r.cost will run for a very long time. Your large map has forced r.cost
to go into a memory swapping mode that utilizes very little CPU time and gobs
of disk R/W time. I played with r.cost this last winter to find some
techniques for significant speedups. Unfortunately I know of no stats for
running r.cost on maps as large as 4.8M pixles. I will however be making
the latest version available on moon soon.