I have 2 lilly plants in my pond and they cover about 40% of the surface area. The largest has been growing for about 5 years, and actually 'grew' out of it's original plastic pot.

They grow quickly.

Some waterlily types are extremely robust, chug out a lot of pads and can smother an ornamental pond too fast

Others have far more sedate growing habits with high pad to bloom ratios, more suitable for ornamental ponds

Many odorata hybrids and in particular tropical waterlilies are by nature brutes that will become 10-15' wide etc...

How well a waterlily can leaf up will depend on its cultivation, some things that can cripple or destroy waterlilies include: salt, poor fertility, excess fertility, fish and critters grazing on roots, excessive heat, crown rot.

Do you thin them every couple years and fertilize them every month during the summer?

Hi
I'm sure climate has a lot to do with how they grow .Tropicals certainly respond to cool weather by more or less going dormant. I usually must repot by late Jan or the entire surface will be covered 3 deep.lol. Can only keep one in the 5x10 above ground due to the aggressive growth. Last Jan for example had 15 separate plants from the single pot!! The only other plant i have room to grow is W. hyacinth but that requires removing at least a bushel a week even then they stack up but wow are they a terrific water filter !!! I guess the trick is to experiment with different methods until you find the one that works for your climate.?? gary