The deduction for BSU savings is 20 percent of the annual savings that you deposit, and is given as a deduction from tax. Every year up until the year in which you reach the age of 33, you can save up to NOK 25,000. You can also deduct 20 percent of the savings (up to a maximum of NOK 5,000) from your tax every year. This is done automatically and will be entered in your tax return. See the bottom of the tax return. No tax deduction is given for the year in which you withdraw the money.
You will find the rates here.