Chapter 9, p.308 (Definitive preview PDF), p.112 (5e core), under the Momentary duration:
Ritual Creo spells with Momentary Duration create things that last as any other thing of that type. The magic is gone in a moment and so cannot be dispelled. This also applies to Ritual healing spells.
Chapter 9, p.315 (Definitive preview PDF), under Ritual Spells
If the spell is a Momentary Creo spell creating a lasting thing , it must be a Ritual.
(the equivalent heading on p.114 of 5e core does not have that bullet point.)
The "ratio" is accordingly however many pawns of vis it takes to cast the spell (that is, the Base Individual of Aquam involved and what the Target and size of the spell are).
Note that despite some statements from other posters, exactly what happens if you use (say) Moon duration magic for food and/or water is not unambiguous; there are semi-contradictory statements in the rules, and different Storyguides prefer different reconciliations. A forum poll in 2006 on magically-created food demonstrates the wide spread of opinions, none (at least then) commanding a majority. Definitive uses the same text as quoted in the first post of that thread.