1) Is it obvious or perceivable that your Astrally Perceiving? (By someone mundane or not perceiving/astral)
You're staring at something. That's it. Someone who knows you can do it, might guess you are, but you might just also be checking out that sweet ride, be distracted by someone hot, or in AR society just be distracted by cool AR meme or jingle. In an AR accustomed society staring off into space is probably fairly normal behavior.
2) It seems that "Astral Perception" and Assensing" are the same roll so you use your Assensing roll for the hiding aura
Astral Perception is the act of seeing the aura, Assensing is reading the aura. Seeing the Writing on page is not the same as reading the writing on the page. They aren't really the same. You could Perceive astral without Assensing someone. There are things to see in Astral.
3) How much can do you allow to be seen just by perceiving an aura?
So perceiving astral is like looking at the forest. You see a lot of trees. It doesn't tell you anything until you start focusing down.
But it will tell you stuff like, what's over there. Presence of Spirit, Wards, Mana barriers, Magic Amor probably, Non-normal BGC, Voids, General Emotional tone of the area. Folks with the bright aura flaw, Anything that weird would probably stand out.
4) Is it obvious you are assensing something? (Are you "staring" at it)? And is it obvious/not on both planes?
See above on 1. Someone actively astrally watching, you could probably tell when you went dual natured, I'd give'em a roll. But from general observation no. All they would see is the character looking at something.
5) When you assense does it take time? How much? (This relates to #2)
If you just looking at one person it should fast, if your taking time to study a whole area, it could be awhile. I'd go with Kiir on that.
Edit: Stupid phones.