Aside from the ending, I find it funny how during the entire duration of the game Earth is under attack. So when you're busy doing side missions or romancing Liara/Ashley/Miranda/etc. or just lounging around, Earth is being destroyed. Earth is sure a lot more resilient than the other planets that fell to the reapers.
That's because the reapers are systematically rounding up as many humans as possible to convert them into reaper slush. IIRC the codex says it's a process that could take decades. If they wanted Earth "destroyed" then they could have done so fairly easily with a mass impactor bombardment, but they only tend to do that to planets with too small a population to bother with.
As for Shepard "lounging around": keep in mind that the game seems to span something like six months to a year (hard to tell exactly) and in that time Shepard goes from one battlefield to another with barely enough time to catch a breath in-between. Indeed in all that time the crew gets exactly *one* shore leave. I don't know what the real world regulations are for how frequently personnel involved in heavy fighting need to be rotated off the front, but I think it's a safe bet it's a little more frequent than that.
People need a pressure valve or something inside them will snap. Hell I remember the stories my Nan used to tell about yank soldiers on leave during WW2. She was a nurse at the time so they mostly involved her treating the ones that got drunk and got their arses kicked.