"He's going in a straight line, ...I measure 625 nm in a straight line"
Not unless he has a motor boat. Remember, the wind is usually coming form the wrong direction and if he has to tack all the way, 625 miles becomes 625 times 1.4 (angling off for the tacks) or 875 miles for the shortest course run.
If he's in a 33' boat and presumably running fairly slow (and somewhat off course) at night or whenever he sleeps? Could easily exceed 900 miles at an average of 4-5 knots...225 hours or ten days run, and that's without any problems from weather or anything else. Like standing off for daylight, instead of trying to enter Bermuda at night.
So without knowing him, or the boat, or checking the wx and stream...I'd say ten days would be time to worry. Nine days would be time to start getting information together and confirming who to call at the USCG if he didn't show up on the tenth day. (Assuming he had provisions & water for 10+ days.) Did he file a specific float plan with you? With intended course and speed, etc?