I believe it's a 20 to 30 minute respawn. I recall waiting many hours for boss spawns in the open world in older MMOs. This is a bone thrown to the fans of spawn camping. You can either use /say to ask the pubs nicely to save you one while you get ready, or you can wait until you are all ready then pounce when it spawns.
The problem with this - specific to Dreadtooth - is that he gets harder every time he respawns. As I understand it, the only ways for him to drop down to his base difficulty are for Section X to empty out (which never happens) or through the Tuesday weekly reset.