According to the law extension of 90 days of Schengen visa or Schengen stay without a visa is not possible.
Short-term stays. The maximum duration is 90 days in a 180-day period. There is a list of countries that are not required to have visas for staying in the Schengen area.
Regarding the short-term stays, you can find more information here.
A visa-free stay in the territory of the Czech Republic is possible because of the bilateral agreement between the Czech Republic and third countries. This agreement applies only to the territory of the Czech Republic. Due to this agreement, it is possible to stay even after the expiry of the maximum length of stay 90/180 days.
The length of the permission to stay depends on the wording of such agreement:
1. Maximum 3-month visa-free stay
Once the maximum permitted stay of 90/180 expires, foreigners should leave Schengen Area. It is possible to travel through the territory of a member state which has a bilateral visa exemption agreement and still applies this agreement over and above Schengen rules, such countries are:
- Republic of Korea
- Costa Rica
2. Maximum 30-days visa-free stay
Under a bilateral visa exemption agreement between the Czech Republic and Singapore, citizens of that country can stay visa-free in the territory of the Czech Republic maximum of 30 days.
3. Maximum of 3-month visa-free stay in a 6-month period
Foreigners can stay in the Schengen Area no more than 30 days and are obliged to leave it for a ]6-month period.
It includes countries such as:
Here you can find more information!