It's incredibly low. I know I was well over 5k kills combined on mobs that dropped it (both pre-cata and post-cata) before I got the pet. I think counted I had close to 5k, so probably closer to 6k or 7k including uncounted kills.
Zone drops tend to take much longer than pets that come from specific mobs, so you could be in for a long grind.
I farmed for 2 to 3 hours no drop. A friend came along to help, he got the drop in 15 minutes. I don't use one of those kill counter things, so I'm not entirely certain how many mobs died. I also leveled skinning there on my hunter and druid. I was probably there for 3 hours total and no drops.
@ Gank, the last Crimson Whelp was listed on your AH 3 days ago. The last unlearned Tiny Crimson Whelp was listed on the 21st of March. Make sure you check both pet sections of the AH since some of the pets can be listed without learning and caging.