The timing for Spells that secure dice has been updated to make it clear that you can choose to use the Spells that allow you to hold dice, after you roll. The FAQ agrees with this as well, but in any case all versions of the rules make it clear that held dice only remain on the spell card until they are used to complete a task.
Spells that hold dice are cast after a player’s roll, not before. The player looks at the roll, then, if he wishes to save a particular die, casts the spell and places the die on the spell. The ‘held’ die remains on the spell card until used to complete a task or until removed (as described below).
The revised rules say:
To cast such a spell, the player places the card faceup in front of him after rolling dice during his turn. Then he chooses one die from that roll. Without changing its result, he places the die on the Spell card’s securing icon.
When a die is secured on a Spell card, it remains there until any player chooses to use it to complete a task, removes it from the spell to roll it (see “Removing Secured Dice” on page 14), or until another effect locks that die (see “Locked Dice” on page 12).
For spells that secure multiple dice, the player may remove dice from it one at a time or all at once.
The original rules say:
Spell: Most Spells allow a player to place the spell card next to the play area immediately before any roll of the dice. Then, after the player rolls, he chooses one of the dice and, without changing the die’s result, places it on the Spell card on the following icon:
While a die is on the Spell card, it is not re-rolled when the active player tries to complete a task, but the die’s result may be used to meet a task’s requirement. Only one die may be placed on a Spell card (unless otherwise indicated on the card) and a die on this card remains on it until it is used
to complete a task.