255 characters comes to mind... but I'm not sure.
a quick google search on "maximum querystring length" brought up
and following a link from another site google showed
from http://support.microsoft.com/default.aspx?scid=KB;en-us;q208427
Internet Explorer has a maximum uniform resource locator (URL) length of 2,083 characters, with a maximum path length of 2,048 characters. This limit applies to both POST and GET request URLs.
If you are using the GET method, you are limited to a maximum of 2,048 characters (minus the number of characters in the actual path, of course).
POST, however, is not limited by the size of the URL for submitting name/value pairs, because they are transferred in the header and not the URL.
well... I guess I was very wrong about the 255....
really, it depends upon the size of you're browser's address bar. netscape 4 could apparently handle up to 30,000 characters, but 6 doesn't get much past 9000