100 characters.
This is the var (variable) in varchar: you only store what you enter (and an extra 2 bytes to store length upto 65535)
If it was char(200) then you'd always store 200 characters, padded with 100 spaces
See the docs: "The CHAR and VARCHAR Types"
Actually, it will takes 101 bytes.
VARCHAR means that it's a variable-length character, so it's only going to take as much space as is necessary. But if you knew something about the underlying structure, it may make sense to restrict VARCHAR to some maximum amount.
For instance, if you were storing comments from the user, you may limit the comment field to only 4000 characters; if so, it doesn't really make any sense to make the sql table have a field that's larger than VARCHAR(4000).
Source: Stackoverflow.com