title.
According to Serebii:
Depending on the level of the island, the Pokémon gets 1, 2 or 4 EVs respectively every play session, so at Level 3, 4 every 30 minutes meaning to get 252 you'd need to have 63 sessions lasting 31 and a half hours. Poké Beans will speed this up so that the play sessions only last 15 minutes per session.
So 99 sessions on a Rank 1 island yield 99 EVs, 99 sessions on a Rank 2 island yield 198 EVs and 99 sessions on a Rank 3 island yield 396 EVs.
Hope I helped. :) Source: http://www.serebii.net/sunmoon/pokepelago.shtml