Here is another way to handle that other than dropping prolonged:
Use the first degree for all difficulties and add a ritual, which means: Quick, 3 seconds, Fast, 10 seconds, Slow, 1 minute and Prolonged, 5 minutes, then add in a variable for Rituals.
I agree that the times are a bit weird considering some spells, but I don't find that one aspect of Atomik Magick/Grimoire being a problem, especially since you are encouraged to use what works for your game. Having 400 spells and a system to cast them is pretty darned handy, especially for the price.