Senko |
I've always treated it as an hour since that never changes based on number of spells e.g. a 1st lvl wizard with 3 cantrips and a 1st level spell takes 1 hour to memorize all of them or less time for part of that to a minimum of 15 minutes. Likewise a 20th level wizard with 4 spells of every slot still takes an hour to memorize all of them or less time for part of that to a minimum of 15 minutes.
Now if your multiclassing to non-arcane classes what happens then? Same rule 1 hour or less than that to 15 minutes. Again the number your preparing increases but only the amount you can memorize in the minimum time changes never the maximum time spent memorizing.
So for me the 1 hour or less than that to a minimum of 15 minutes applies for spell preparation regardless of the number/type of spells being prepared. A case could however be made by a GM that it is 1 hour for arcane and 1 hour for divine as the divine preparation is spent praying to your diety/spirit/other and the arcane is spent on meditation and inward reflection. I never felt doing that would add to the game though.