A down bag will get progressively more damp every day and night you spend in heavy rain and fog (100% RH). In my experience, if it's raining and foggy all day and you have no way to dry it out in the sun or a machine, it starts getting really noticeable on night three, and by night four you have a very damp bag but still survivable. This has happened several times to me, every time on trips through Washington's North Cascades. Every time I was happy to have had the down rather than synthetic. The nice thing about bike touring is that you can usually get somewhere dry and warm within four days and take care of your bag.