ASHRAE -- the American Society of Heating, Refrigerating and Air-Conditioning Engineers -- recommends maintaining a relative humidity (RH) between 30% and 60%. Publications from the US Environmental Protection Agency recommend between 30% and 50%. To give these numbers some context, consider that desert air has a relative humidity of about 25%, while a heated house on a cold day can have an RH of about 15%. Also, the relative humidity of your home can vary from area to area, especially if you have a basement or areas with limited air circulation.
The humidity level you can attain in winter with a humidifier may be limited by the quality of your windows. You don't want condensation on your windows to form and drip and ruin your woodwork. If you have cold, leaky windows, this may occur before you reach your desired humidity level.