No official student-to-citizenship timeline exists. Build your own from these fixed rules:
You become eligible on the later of two dates: when you reach 4 years of lawful residence, or 12 months after your PR is granted (Home Affairs residence calculator).
Example: a 2-year Master's followed by a 485 means you reach 4 years' residence during the 485, but you still need 12 months as a PR. The real driver is when you obtain PR, which depends on your occupation, points, nomination or employer - none of which is guaranteed. Home Affairs publishes current citizenship processing times; check them when you are close to applying.