There is a side-note, or something similar in the Spellcasting section near Veils suggesting to simply (even as Evocation) run them as lasting the duration of a scene, or until pierced.
Enchanted Items, I think, can be imbued with Thaumaturgy spells as well; which can last the duration of a scene. That may only apply to potions though, I don't have the books to hand.
Either way, I'd probably just run it as a Good (+3) Veil, that lasts the duration of scene. Beating its rating with an awareness, or lore (arcane perception type thingy) check pierces the Veil allowing them to notice you. Likewise, if you make any obvious actions that'd draw their attention, you become noticeable (e.g, attacking them).
Those are my thoughts on the matter, at least.
To get a stronger Veil then Good, or to increase its uses per session, you'd have to increase the number of Enchanted Items slot used. That is, working under the assumption you've no specialisations in crafting, as you didn't list them.