I have 2 questions on spell duration for the Dresden Files RPG:
Is it possible to cast a shield spell that has no duration as a rote, but then prolong the shield on my next turn?
Can you prolong an evocation spell like Fuego?
if i remember correctly you can cast the shield on your turn, and then once it comes back around to your turn you can spend your entire turn to put shifts into the shield to make it last. the shield, i believe, is considered an evocation so it should be ok to use it as a rote spell.
i don't think you can prolong an attack evocation.
when i get home this evening i'll do some readthrough in the book to double check everything.
You can prolong any evocation by gathering shifts of power and putting them into the evocation in the round after you cast it. As the text says (YS p. 259), doing this is rules-wise identical to casting a new spell, except that the power of this "spell" is how many more exchanges the original spell-to-be-prolonged will keep going. You get to keep the original spell's rating.
For a spell like Fuego, that means you keep the original weapon rating, and just keep it spewing like a flame-thrower. As a GM I might require a new Discipline roll each exchange for targeting purposes.
For a shield rote, it works exactly like the example on page 259 of Your Story where Harry throws up a zero-duration shield and then prolongs it. The difference between doing this with a rote and with a normal evocation is just how easy it is to cast – the difference is irrelevant to being able to prolong it.