In general as a EEA national, you have the right to stay in all EEA countries, no matter how long if fulfilling certain conditions. However some countries ask you to report your stay, even if it is shorter than 3 months. As a general rule in the EU the same requirements apply to citizens of those countries as well.
These countries according to the EU job mobility portal are:
- Belgium You are required to report your presence within 10 working days at the local town hall.
- Czech republic You are required to report you stay within 30 days of arrival.
- Germany You are required to report your non-tourist stay within around one week after arrival at the local Meldebehörde. The exact details vary by state and can normally be found on the cities' website.
- Slovakia Your are required to report your stay within fifteen days, even if staying with friends.
- Switzerland You must register within 14 days. Note that Switzerland may be imposing additional restrictions in the near future.
Although it is not explicitly stated on the site, you don't need to report your stay in Denmark.
You might be exempt from this rule if you stay for touristic purposes (as in Switzerland) or for a shorter time (less than two months in Germany).
Sometimes it may be beneficial to register your stay even if it is shorter than three months (for example if you want to work or open a bank account).
Please note that these are just formalities and the countries are required by EU law to issue all documents and that can only be refused in some serious cases (such as imminent danger for national safety or health).
For stays longer than three months almost all member states require registration and some sort of documentation, sometimes even just after arrival if you intend to stay longer.
In general you cannot be expelled for not following those rules, but fines can be imposed.
