While this effect is not that powerful and is symmetrical, it will last 2 turns. Keep in mind it works for your commons also when they are adjacent to your opponent’s summoner which makes it a usual finisher of games when you want your commons attacking the enemy summoner.
The most frequent use of this event is to punish a player who has their summoner hidden and / or doesn’t have commons and then you can use this event two turns with relative impunity as long as you position Valeria correctly.
Both Law of Liberty and Loyalty require commons for your opponent to take advantage of, but commons are something for which High Elves have great tools to punish with Valeria, Oblige and Arbiter of Justice. So if your opponent is not using many commons which makes those key cards not as effective, try to use both of these laws to take advantage of that.