That will depend on where you are coming from and where you are going to in Europe. If you are from a European country, then you can stay there legally for all of your life. As for visitors, different countries in Europe have different rules. Some countries have more stringent rules for people coming for particular countries than they do for others. It is easy for some people to get a visa than it is for others. There are also different kinds of visas. There are also other things, like if you were married to someone from a European country and you were not from Europe yourself. So there is no single answer to the question, as there are many factors to consider.
Citizens can legally stay forever.
Permanently, if you emigrate there.
90 days
three months
For the rest of your life, unless they are legally expunged.
Ask a long stay visa
As long as it has been legally declared by a court. It may be for a month or for life.
I will say about since january
In Western Europe from about April to September.
How ever long you can stay awake
They stay with you forever. You must legally report them forever.
to stay there legally, permenantly you have to pass a citizenship test