have been killing snakes for over two hours no charm yet they also drop green snake eggs, sand snake eggs, green snake scales, green snake meat, sand snake meat
The charms are by no means common . . . if they were, I would have a pile stacked high . . . I regularly kill mobs in Undershore on the order of hundreds and see the charm very infrequently given the ratio of drop to kills.
Two hours of killing (and this assumes no downtime at all) isn't even an average warm up for this or any drop in Undershore if you are looking for it. Basically, go kill in Undershore if it gives you good experience, and if anything like the charm does drop, its a nice bonus. Farming in Undershore for stuff like the charm is very much like farming in Natimbi; it can be a long wait between drops, then may not be the drop you want.