post grid excerpt

hi
I create a specific item design in post grid, I want to customize post excerpts words count, it shows now 60 words, but I want to decrease it to 20 words
what should I do?
I searched so much, I add filter in function.php in my child theme,

add_filter('avf_postgrid_excerpt_length','avia_change_postgrid_excerpt_length', 10, 1);
function avia_change_postgrid_excerpt_length($length)
{
   $length = 10;
   return $length;
}

but it doesn’t work.