Absorption rate of Infrequently eating animals?
I'll take a stab at this.

What fraction of your food is absorbed and what fraction is expelled is a function of what kind and how much indigestible material there is in the food, and not related to how often the animal eats.

How often an animal eats is a function of its metabolic demands(how many calories it needs) and how many calories it can get in one meal. Reptiles, as ectotherms, have a lower metabolic demand than endotherms. In addition, snakes can swallow huge amounts(relative to body weight) of high-calorie food in one sitting. As a result, snakes eat infrequently. Pet snakes are fed weekly or biweekly, depending on a number of factors. A reticulated python can swallow and digest something up to its own body weight. Ten weeks of digestion gives 1.4% of its own body weight daily, even without fasting.

Elephants are a migratory large mammal and eat about 4% of their body weight each day, more or less. This takes about 12-18 hours to collect, according to these people. 

Elephants eat high-fiber, low-calorie foods, and as a result about 20% of their food mass is turned directly into poop. They feed often, and don't absorb much. Hummingbirds eat constantly, consuming their own body weight in food each day, but poop almost not at all. Their feces contains the indigestible parts of their food, just as the elephant feces does. The different is that large parts of elephant diet is indigestible fiber, while a tiny part of the hummingbird diet is indigestible insect exoskeletons.

All creatures absorb as much nutrition from their food as they can, and expel the remainder. How much remainder there is depends on what kind of food they eat. 

Growth rates versus digestion rates and mass intake is a totally different and much more complicated question. Generally speaking, the growth-limiting factor is DNA replication or an essential acid or mineral deficiency and not a raw mass problem.