Depends on the weather. I keep all my neps in a south-facing conservatory which heats up a lot when it is sunny, but stays very cool when it is cloudy. On a hot, sunny day, both humidifiers can drain their 4 litre tanks in a day. With more typical Welsh weather, they can last a week.
Incidentally, I've been using fairly hard tapwater in them for over two years with no problems. I just clean the plate with the little brush that came with them every now and then.