1) Doing my first really long trip now (tens of thousands of light years is kinda the plan), I stop every now and then, quite randomly really. I've had to learn away from my habit to scan everything and just ping&go, otherwise I wouldn't ever be able to reach these long distance destinations. The way I'm going right now I think I'm doing a full scan roughly every 10-20 systems or so.
2) I survey everything except asteroid belts basicly (and sometimes even those). I try to restrain myself, but usually when I'm near a gas giant (for example) I just start scanning the moons, and when I've done that for one giant, I need to do the rest as well. And when i'm done with the primary star, of course i must go to the rest of the stars 400k ls away as well...
3) No rank increase in wilderness. Only when you get back and cash in. It is unknown if the rank is solely based on the amount of credits made from the data sales, or if there's other factors included (such as # of objects scanned, % of objects in system scanned, or other such things) - however since these things (quality of data) are (supposedly) represented in the credits received for turn-in, one could assume the rank is solely based on the credits received.