Archive limit the text of the_content

I’m assuming ‘Archive Page’ is a custom template and not either of the WP archive pages (category/date/author/tag/taxonomy). Try using this:

<?php
add_filter('the_content', 'trim_content');

function trim_content($content)
{
    if(is_archive())
    {
        //use your own trick to get the first 50 words. I'm getting the first 100 characters just to show an example.
        $content = (strlen($content) <= 100)? $content : wp_html_excerpt($content, 100);
    }

    return $content;
}

I think this will do the job.