Allen's brew is temporary, though better than the Ultimate Full Resistance potion (boost of 36, rather than 23); I assume it would have the same effect time as other similar potions, but picked the other alchemist, and only used a couple temporary boost potions at the start of the game.
Allan does have a couple quests for you in the battle tower, though he wants a wisdom potion for his second quest that gives a permanent +5 Int boost, which you can drink yourself if you pick Barbatos.
