The amount of EVs gained per session on Isle Evelup depends on the level of the island. I'll assume that you're referring to a level 3 island; and in that case, your Pokemon will gain four EVs per 30-minute session on the island. (At island levels 1 and 2, they will gain one EV and two EVs respectively.)

The playtime assignment screen you're referring to lets you choose *how many* 30-minute sessions you want your Pokemon to do. If you max it out at 99, then your Pokemon will receive 99 * 4 = 396 EVs, which will take 49.5 hours to complete in total. Of course, you only need 252 EVs to reach the maximum, so you can set 63 sessions instead, for a total of 31.5 hours. You can then halve this to 15.75 hours using PokeBeans.

As you have probably deciphered already, the Isle Evelup is a very inefficient way to train EVs. Your PokeBeans are better spent on the Isle Aplenny.

Reference for EVs gained