demanding
v.v. needing a lot of time, energy, or attention. You use this to describe a person who is hard to please or a task that is very difficult.
v. requiring significant effort, skill, or patience; characterized by high expectations or rigorous standards. When applied to people, it implies a tendency to make frequent or difficult requests.
My new boss is very demanding and expects perfection.
The medical degree is incredibly demanding, requiring students to study for twelve hours every day.
While the client was notoriously demanding, the design team found that his high standards ultimately pushed them to produce their most innovative work to date.
From demand + -ing.
Typically functions as an attributive or predicative adjective; it is a gradable adjective that can be modified by 'very' or 'extremely'.