From MSDN:
A GUID is a 128-bit value consisting
of one group of 8 hexadecimal digits,
followed by three groups of 4
hexadecimal digits each, followed by
one group of 12 hexadecimal digits.
The following example GUID shows the
groupings of hexadecimal digits in a
GUID:
6B29FC40-CA47-1067-B31D-00DD010662DA
From Wikipedia:
Often braces are added to enclose the
above format, as such:{3F2504E0-4F89-11D3-9A0C-0305E82C3301}
So a total of 38 characters in the typical hexadecimal encoding with curly braces.
-Adam