Is there a modifier for permanent spell durations in Ars Magica 5th edition?
Hermetic spells cannot ordinarily create permanent magical effects. This is the Limit of Essential Nature (Ars Magica 5th Edition, p79). However, it is possible to permanently create objects using Creo magic (AM5e, p112). Spells of this kind take the Momentary duration, i.e. no increase in level beyond the basic effect, but must be cast as a ritual as per the ritual casting rules (AM5e, p81).
You can create an enchanted device with a constant duration,
or, if you are ok with your spell being stationary, you can make it duration ring and it will last until the circle you've inscribed around the spell is broken. (It is not explicit that duration ring spells can not move but, for game balance reasons, I've seen more story guides decide that they can not than decide that they can.)