What's the correct word for "demandingness"?

So, if your promotor is kind, you would thank him as:

"I thank Prof Smith for his kindness."

Now, what if your promotor is demanding, as in tough and exigent when it comes to work? How do you express this quality?

It seems to me the following is not correct English:

I thank Prof Smith for his "demandingness".

Which is the right word to replace "demandingness" above?

If it could help the forum choose, I need here to convey also the impression of "at times excessive but on the whole beneficial".

Solution 1:

If you want a noun,demandingness is a correct term:

• the quality of being demanding.

(Collins)

or:

• You may thank him for being demanding, rigorous or exacting.