Evo veils last one exchange. You can push more shifts of power into it to make it last longer though. If you do so when you first create it you will have to make a weaker veil though. However, you can cast a strong veil on the first exchange, then extend it for multiple exchanges on the next exchange, then do other things on the subsequent exchanges until it runs out of power, then you could still take an exchange to re-power it for more exchanges again if you liked.
I would imagine that if the veil, evo or thaum, has duration left then it does not break when you attack someone. However, they may have other ways to find you once they know you are present (the sight for instance), or they may be able to attempt to defend or attack still if you are nearby, but have -2 shifts to everything they do because of the veil aspect on you. This game leaves a lot of such things up to the players and GMs, so if you think such a thing is too overpowered in your campaign then just make it drop. If not, keep it in. If uncertain, then make the veil drop until the start of the players next exchange, so the person attacked could just delay his action until you attack, then retaliate before you veil up again.
