How long, it usually takes to hear back after the campus visit for assistant professor position in Netherlands?
I don't think there is any "usually" here - it will depend on multiple factors (including whether you were the first or last candidate interviewing, how many layers are still involved in making the final ranking, etc.).
On the bright side, I think it would also be perfectly fine to send an e-mail to your contact person at the university to ask whether they have a timeline for the decision making. No one will be offended or think that strange I think, if you just mention that you are curious.