They take several years to reach a size that could be described as monstrous, provided they can overwinter without much damage. The larger the stock that you got, the quicker they become large plants. I have one that grew to 10 ft. with 5 ft. diameter leaves in about 5 years. Another with a stock about half the size of the first one is 10 years old and it bearly reaches 7 ft. with 4 ft. diameter leaves. This is a common plant in my area and I know of several which are over 50 years old. They love heat and humidity and don't seem to need much, if any, fertilizer.
I cover mine with its own leaves and stick the stocks through them to ensure the wind doesn't blow them away. I then cover this with deciduous leaves and a layer of sand or dirt on top of this. Works for me but our winters are not severe.
They are truly majestic when large and I feel, well worth the time and effort it takes to grow them.
Good luck.