10-20 grams/month.

A weight loss of more than 50 grams makes sensor restoration difficult, and beyond 100 grams, it is virtually impossible. Weight loss is affected by ambient humidity, flow rate, and in certain cases, the type of sensor being used.

The sensors work best, and last longer, if they are kept moist. If your sensor is losing more than 50 grams of water per month, add water every two weeks.