Based on math I did elsewhere, I’d estimate water loss due to complete humidification of 0%RH inhaled air as about 21 grams per hour, less than an ounce. This assumes you’re at rest (e.g. stuck in an economy class seat). If you’re exerting yourself (e.g. you’re in a struggle with the Federal Air Marshal after having angrily demanded yet another glass of water)), then your tidal volume and breathing rate will be considerably higher, and so will your water loss rate.