So far from the anime I remember there is a Class A and a Class B for students, and in episode 24th from season 3, 3 students from a superior year appeared to give a talk to the students from Class A. How many years do hero students study at U.A. until they become professional heroes?
According to web-japan.org (emphasis mine):
The basic school system in Japan is composed of elementary school (lasting six years), middle school (three years), high school (three years), and university (four years).
Considering that U.A. is a high school, we can conclude that students study for three years before graduating and becoming professional heroes.
However, it may be important to note that in Season 3, Episode 22:
Class 1-A was able to obtain their provisional hero licences, allowing them to do limited work as pro heroes.
Knowing this, this could change the answer to:
Under 1 year.
I'll leave it to you to decide what the answer is.