A creature in my story was largely inspired by the xenomorphs of the Alien franchise. In wondering how long it would take one of these creatures to grow from conception to maturity, I decided to look at my original inspiration which only posed more problems. Xenomorphs develop insanely fast! Growing from an egg to a nymph in a matter of hours and from a nymph to an adult drone in maybe days. So my question is as stated: How fast could a large creature feasibly grow and how much would it need to eat?
- How fast could a large creature feasibly grow?
Growing from an egg to a nymph in a matter of hours
That given the size of the alien you reference that pops out of the egg is perhaps the most implausible part of the the thing as growth of embryos in animals is not pounds or ounces per minute or hour but rather cell divisions per "x" period of time, & you start with a single cell.
and from a nymph to an adult drone in maybe days
While this given the starting size of stage in it's life cycle when it bursts from someones stomach is perhaps more plausible, if we assume a level of cell division comparable to the early stages of cell division in a fetus rather than in juvenile animals after birth.
Duration of the Cell Cycle "Certain fly embryos sport cell cycles that last only 8 minutes per cycle!"
If we assume this is about as fast as it gets you could perhaps be looking at exponential growth (a doubling in size) every eight minutes.
- and how much would it need to eat?
A bit more than it's own body weight every eight minutes.
Which may be a problem for a large complex multi cell organism as you'll need
- An extremely efficient stomach & gut to absorb all those nutrients & calories fast enough.
- An extremely efficient cardiovascular system to get it all to cells that need it quickly enough.
- Extremely efficient lungs for oxygen absorption to power the cell processes you're feeding.
Which may all need to be implausible efficient to sustain anything like that speed of cell division in large organisms, there's always hand waving though & you've not included a reality check tag.
The organism is going to need a certain number of calories just to sustain itself, I've had difficulty finding details on this so I'm going to have to extrapolate from Google searches on human statistics.
The average calories needed by an adult male is 1,800 calories a day.
The average adult male weight is 197.6 pounds
Extrapolating from those search results (which is of course wildly unsafe for all sorts of reasons, but all I have) gives a daily figure 9.1 calories per pound of body weight just to sustain itself.
You should consider the above more along the lines of my showing my workings for a math question rather than my answer, gotta go for now so that will follow later, possibly a lot later, sorry about that :)