There are two options, really:
- A buff applied now applies for the rest of this turn, and the enemy’s next turn, and expires at the start of your next turn
- A buff applied now applies from the start of the enemy’s turn, through your next turn and expires at the end of that
Which of those two is better depends a lot on the style of your game. In general, the first gives a more ‘rapid action’ feel, as it lets you ‘combo’ moves by applying buffs within the character pack that are applied immediately and modify the abilities of that character.