how to enable different number of revisions for different blogs when using WPMU?

for example i would like blog1 to have at most 3 revisions for each post
but i want blog2 can have at most 10 revisions since this blog may have more than one editors that i want to enable this blog to have more revision than blog1, how to do this?

wp-config.php only support for global revision, and can not be customize for each blogs, so should i implement this function by myself?

No, wp-config.php can have granular constants. Like this:

if ( 'example.com' == $_SERVER['HTTP_HOST'] ) 
{
    define( 'WP_POST_REVISIONS', 1 );
} 
elseif ( 'blog1.example.com' == $_SERVER['HTTP_HOST'] ) 
{
    define( 'WP_POST_REVISIONS', 5 );
}
elseif ( 'blog2.example.com' == $_SERVER['HTTP_HOST'] ) 
{
    define( 'WP_POST_REVISIONS', 10 );
}