The deadline
You have six weeks (art. 6:7 General Administrative Law Act), starting the day after the decision was made known (art. 6:8(1)). A late objection is usually declared inadmissible unless the delay is excusable (art. 6:11).
What to include
Your name and address, the date, a description of the decision and why you disagree, and your signature (art. 6:5). If something is missing you're usually given the chance to fix it (art. 6:6). Short on time? File a brief objection on time and ask to add your reasons later.
Special cases
Taxes and benefits also have six weeks. Traffic fines under the Wet Mulder follow their own rules: you appeal to the public prosecutor. Lex checks which rules apply to your letter.