Reasonable length of time for a warranty period?

The whole point if this it to present a customer with a safety net to make them feel confident about purchasing an animal sight unseen and being exposed to the stress of shipping.

That's what it boils down to Rich, customer confidence. As a seller I try to instill that confidence in the buyer, but as a buyer I have a difficult time feeling confident of someone who limits their guarantee to 24 hours regardless of whether they claim they are lenient in applying it. The fact remains that it is an easy out if a problem does arise.
As for live arrival only, this pretty much eliminates any confidence whatsoever, and I simply pass on purchases under these terms.
It should be noted though, that for large quantity wholesale orders, I do find that acceptable, and I do not offer an extended guaratee in those cases. The much better price given to wholesale purchases along with the common conditions of such people's facilities in many cases does not warrant an extended guarantee.

For some animals, the metabolism is much faster, and with some exceptions, the animals with slower metabolism seem to develop problems at a slower rate. I could be totally off base with this, as I am only speculating, having the vast majority of my experience with Beardeds.

You are right on that Richard. While chameleons and certain other lizards can go downhill very quickly, snakes can easily linger on with problems for an extended period.
My comments and policies are for the most part derived from the perspective of a snake breeder. I will say that offering a 30 day guarantee on a chameleon may well be excessive, because in a months time husbandry can kill a cham.

You also mentioned cost effective for the seller. This is the area that creates the difference in policy between breeders and dealers. I most often notice it is breeders that offer extended guarantees, while dealers like to stay at 48 hours or less. This is a broad generalization and there are of course exceptions on both sides.
Dealers who breed nothing have an monetary investment in everything they sell. This gives reason to limit guarantees. Breeders on the other hand have a different sort of investment in their offspring. While it did cost money and alot of time to produce that animal, it's difficult to assign an exact figure to it, and replacing an animal doesn't really equate to a negative return. On the contrary, replacing an animal will likely lead to selling another.
Breeders are also intimately aware of the state of what they are selling. Having had it since birth, they know whether it is likely to thrive or if it may be a potentially problematic one, whereas a dealer may only have had the animal a few days when they ship it to a customer. For this reason they are in a position to offer the longer guarantee without assuming as much risk.
Where I depend on nothing but my own methods and husbandry to assure me that I am providing a healthy animal, a dealer who has only had the animal a few days has to hope his suppliers husbandry isn't going to cause him problems later. Considering what you brought up about snakes taking longer to develop problems, this adds to the risk dealers take. In my opinion it's just part of conducting buisness they way they have chosen.