There are no automated processes that delete messages - it takes an action by a human to delete messages, either individually or en masse.
The "DaysPrune" in the query string is a legacy term (that we keep hanging onto because we like it <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" /> ) that only changes how many active days of posts are visible. Also, the 1000 just means "show all" instead of actually restricting to 1,000 days of posts.