It takes longer to tame than the timer says because the cool down on the veggie cake is 30 sec and for that time it's like there is no food in it's inventory. It loses about 1% of progress each time it eats because of the cool down.
Mine kept glitching while taming, took almost triple the cakes and double the time (kept saying put food in inventory to tame). I found a way to bypass this by putting a stack of berries and a stack of meat in it's inventory along with cakes while it's knocked out. Hope this helps someone if anyone else has had this problem.
The Achatina is a Knock-Out tame, using the expensive resource of Sweet Vegetable Cake. Using a taming pen is the most suitable strategy, due to the Achatina being found in the Swamps and Redwood Forests, some of the more dangerous areas of the ARK. Plus, because of the size of the Achatina, it is easily able to be carried by all ridable flying mounts, expressing the fact that a taming pen would be the best way to tame this critter.