The cooking time will depend on how thick your tri-tip roast is (the thicker it is plus your level of doneness). The thicker cuts usually need more time than thinner ones because they need more heat to reach an internal temperature high enough for cooking.
The minimum sous vide cooking time will be 2 hours at 134°F and the maximum time will be 4 hours at 140°F. However, if you want to cook it medium-rare, I would go with 2 1/2 hours at 134°F. If you want to cook it medium, go with 3 hours at 140°F. If you want to cook it medium-well or well done, go with 4 hours at 160°F.