It's usually fine for high frequency metros, particularly in peak times with more trains, but time-based zone fares aren't always reliable in other cases.

For example, in most of Switzerland and probably Germany that uses unified zone systems for everything (intercity trains, local buses, boats, funiculars, etc.) the frequency of most train routes is nowhere near those of a typical bus.

Zones aren't usually time based though, they're based on where you got on and how many zones that is away from where you are when your ticket is checked.