the investigation into a report takes a long time, can I extend the deadline for feedback

Three months is the statutory limit, not a target deadline

The deadline for feedback is fixed at a maximum of three months after the receipt confirmation, and that deadline is not something the employer can unilaterally extend. Article 9 of Directive (EU) 2019/1937 speaks of a reasonable period not longer than three months, and that "not longer than" is a maximum, not a target that can be pushed upward if the investigation requires more time. If an investigation takes longer, that does not change when the whistleblower deserves feedback — it only changes what that feedback consists of.

In practice, this means that the feedback within the deadline does not need to be a final decision. An interim update — the investigation is still ongoing, these steps have been taken, this is the expected next step — also satisfies the obligation to inform the whistleblower within the deadline. What is not possible is to stop the clock or stretch the deadline itself because the investigation is not yet complete. Anyone who is unsure about exactly what must happen within those three months will find the explanation about when a whistleblower must be informed about the assessment and the steps for the period immediately after a disclosure in what to do after receiving a whistleblower disclosure.

Where this follows from: Article 9 of the Directive and Article 2 of the Whistleblower Protection Act

Article 9 of Directive (EU) 2019/1937 formulates the feedback deadline as a maximum of three months, intended to give the whistleblower clarity about when they will hear something — not as a minimum deadline that the investigation must wait for. Article 2 of the Whistleblower Protection Act incorporates this obligation for the internal reporting procedure in the Netherlands, without including a separate extension option. Also § 17 of the German Hinweisgeberschutzgesetz, which describes the course of internal disclosures, is based on this fixed deadline as a framework within which the whistleblower must hear something from the organisation. None of the three sources provides a basis for extending the deadline itself in a lengthy investigation.