I'm with Annandaler on this one. 90 minutes should be fine. Keep in mind IAD has only one checkpoint, but there are two entrances to the "premium lines." There are about 20 stations at the checkpoint but I have never been lucky enough to see more than 10 open. Also, they don't always have the premium lines open, and sometimes even if they do they don't enforce the rules. So, 90 minutes may turn out to be far too much, but it is safe in case anything is not perfect. (And at IAD it hardly ever is perfect )