It's not for achievements, it's for items only. Just know that they capped the time you can get drops on a per-week timer. The best estimates seem to be about 15 hours per week. If you're a casual player and you play less than that, then idle once or twice a week overnight and you'll be at cap. If you're an active player who plays 15 or more per week, then you need not idle at all. They instituted this change a while ago to defeat people who were using scripting programs to "farm" items without even being in game. Plus, there's a trading system now, so you can swap items with people for stuff you want or need.