King-eL pointed out to me that it varies not just between individuals, but also varies between different stages in life.
Young endli will grow fast, but eventually their growth slows, and they may hit growth spurts, but eventually its slow growth until they hit their adult size.
I'm working on a progression thread for an endli of mine, and already I have recorded 1" of growth against a ruler, after 1 week!
Now that I have more baby endli, I will track each one to see what variation I can record in growth speed, and come up with a mean measurement.