Magic Spider Filter
Astral projection is fraught with perils, even when you know what you're doing. The longer you stay out of your body, the connection between your astral self and your material body wears thin, and you run the risk of breaking that connection (dying), so you're correct in your assumption, it's a temporary measure. For you, I'd say a good three to five hours, but that's with my supervision. If you stray too far away from your body, you run the risk of floating off into other dimensions, unable to find your way back (resulting in coma). There are also entities that live on the subtle plane from which, as somebody without experience in magic, you are not able to protect yourself. That is why I have a rule to only perform this spell inside the protection of the Sanctum, and that one's astral body remain within the Sanctum's walls.
However, if you wanted to look around town... I would make an exception. But I'd insist on coming with you.