How long can you stay in mercosur country with cedula from another mercosur country?

Well, it depends on which country you are staying and which country your cedula is from. Technically speaking, you can stay forever because of the Mercosur visa agreement. However the implementation is different in each country. The simplest ones to stay seem to be Chile and Argentina. Once you are in either of them you just need to go to the Departamento de Extranjeria and fill the form to get the Mercosur visa. Once that is done you will receive your residence permit in about a week. That permit is valid for two years, but after that you can change it to a permanent residence. In both cases after 5 years you can get citizenship.

Brazil is more complicated, because you need to start the procedure before getting into the country. I don't know about other countries, but I suspect that Uruguay and Paraguay should be easy too.