Too many variables - motherboard/BIOS default voltages etc will play a factor, as will ambient temperatures in the room.
I would think 38-41'C is a bit on the high side if truly 'idle' (ie no Windows update etc), but ~10'C above ambient temps would be a reasonable jumping off point in most situations.
Unless your ambient temp is ~30'C or your motherboard is defaulting to high voltage, it probably sounds a little on the high side.
All that being said, idle temperatures don't really matter at all. What matters is load temperatures, which you can establish by running stress-tests (and with liquid cooling, you want to run those tests for 20-30 minutes, generally to let temperatures reach a steady state).