Practically it can accomadate a very lengthy value which will be sufficient for normal requirements.
A multi line text field is Defined in sharepoint as a 'Note' type, this is stored in the the DB as a ntext sql type.
In the SQL type can store 2gb of char data, that's 1,073,741,823 characters.
Obviously there are practical limits on how big this data should be allowed to get - as SP is a web based system, you do not want to be passing 2gb back and forth all the time.
You can't set it, but you could use a workflow to check the size of the data in the fields and ether truncate or warn about the size of the field.
Since sharepoint is totally based on SQL the limitation of SQL columns maximum length will be the sharepoint column maximum limit.
If you come to sharepoint context, it is number of columns that is based for limitation ,a list can accomodate rather than individual column max limit.