As far as i know its not the government that keeps the data itself, is the ISP's and service providers.. its a minimum period of archiving, for them to respect, so if a law enforcement signed by a judge ask them to reveal some logs for instance, they should have it, if its in the date range they have defined, or else they would also be accountable.
There's a voluntary program in the Netherlands that financially compensates ISPs for uploading logs to a central database where law enforcement is free to query it without, in practice, judicial oversight.
Laws are great, but they're easily circumvented. Especially in the Netherlands. That's why this "Free Internet" project is reasonably useless. Its view is too narrow, focusing only on official laws and not on actual practices. For instance, after the whole Project-X incident[1], law enforcement has been looking into being able to remove social media posts on a whim, without judicial oversight. Such underwater erosion of rights is common in the Netherlands.
And a maximum. Privacy laws also dictate that data should not be retained longer than needed for the purpose for which you're storing it. If you store data for statistics, a few days would be the max because you can count hits and anonymize data almost immediately. If you store data for the government and the government only needs it for 6 months, then after 6 months you have no right to store it anymore. And ISPs really stick to this.
I think this is quite good