ATF is oil so you can leave it in there indefinately and it will not hurt
anything.  The only problem you might run into is that if there are alot of
deposits that have built up in your engine the ATF's detergents will loosen
them up and they could plug up your oil filter.  So just to be safe, I might
consider running the van for 300-500 miles and then changing the oil and
filter (or if you just changed the oil then change the filter only).
Hope this helps you.
Ken Wilford
John 3:16