When you have an Ars Magica character with Flexible Formulaic Magic (FFM) every opportunity to lean a new Range, Duration, or Target adds significant leverage into a character. It may not add game shattering raw power, but instead add breadth of options which is why FFM is such an interesting virtue.
Duration: Tide – effect lasts until the tide changes twice.
I though that Duration: Tide might make an interesting new Duration for a magus. I see it as equivalent to Sun (+2), as the guideline would require the tide to change twice for the duration to expire. This makes it slightly less powerful than Sun during part of the day, and slightly more powerful afterward. A nice match.
A water or ocean oriented Magus could even replace all their typical Sun duration spells with Tide as a quirk of their magic to add flavour. There is an argument to make that use of this duration would require a certain level of magical experimentation and discovery, or that it should be a +3 mag adjustment just to be “harder” than Sun, and if required by the troupe this would be a simple discovery. The counter argument would be to let the Magus create a series of formulaic spells with Tide as the duration, as an ad-hoc way of learning the deeper aspects of tides, and that is certainly already allowed in the new spell creation guidelines of Ars.
As a story guide I’d dread trying to ponder when the tides would change twice, so I suggest using a tide checker site for the story location and take that as a broad brush solution.
p.s. the artwork is a wonderful piece by Bill Sharp. It’s a very clever image, simple on first look, but there is a lot of character to it as you look. Please take a look at the rest of his work.