If you are using a Hydro Carbon wax you can not wax too much, if you are using wax with Fluoro it is possible to over wax, how much no one really knows the skis all of sudden just don't glide like they should. You can use a a special wax remover like Holmenkol's Nano CFC remover that only cleans out the fluro's and does not effect the paraffin on the base. Just make sure your iron never touches the base with no wax on it
you can wax to much but that will only make your skis sticky for the first few runs. I usually overwax my skis anyway because I would rather have to much than not enough.
As for leaving it overnight you don't have to as long as the wax has cooled but some waxs do take longer to cool than others