Rebuilding or regenerating the teaser after changing the trim length in Drupal 7?
If you are using the Views module to display your teasers, and you are using fields in the View, then you need to set the trim length for each field in the View itself.
Are you using custom formatters? These might be buggy and/org disregard the trim length.
When you edit the node(s), check that the summary field is blank. (I quote from the node edit page: "Leave blank to use trimmed value of full text as the summary.")
For further troubleshooting, create a new node with a lot of dummy text. Does it get trimmed to 300 or 600 characters?

If the new node gets trimmed to the new value, but old nodes get trimmed to the old value, then there is a problem with your cache or with the old nodes.
If the new node also gets trimmed to the old value, then there is a problem with your fields or views.