Subtract, divide by effective gain, and round up.
Subtract your current EV from your target, divide the remainder by the effective gain per training repetition, and round up. For example, a target of 252 from 0 at a gain of 2 takes 126 repetitions.
This is a planning calculation, not a database of current Roria Conquest opponents, item effects or hotspots. Examples use assumed inputs, not in-game tests or confirmation of the current EV cap. Define one repetition consistently: one defeated opponent with fixed gain, or a complete encounter with a known total gain. An average across different encounters is not an exact plan.