I’m agree with the comment of @birgire, but if you really want to do it with custom fields, I think it’s better to group all datas for a post in one custom field in an array.
After, you only need to use get_post_meta
to get older values and add the new one in the array. Eventually, you could apply a php function to the array limite to 5 days. update_post_meta
will save it as an array, to get it in your template, just create a display function.