The amount of time required to cook top sirloin roast beef varies from recipe to recipe, but the Texas Beef Council recommends that it be cooked only until it reaches an internal temperature of 135 degrees Fahrenheit. The amount of meat prepared affects actual cooking time.
Since some cooking continues when the beef is removed from the heat source, it can be safely removed when it reaches just 130 degrees. This recommendation avoids any mention of time at all, instead suggesting the chef cook the piece of meat only until it reaches proper holding temperature.
A benefit to not overcooking beef is flavor, and top sirloin tastes best to most people when cooked to what chefs call "medium-rare." This means that some pink color still appears in the inside of the meat. Regardless of the color of the inside of the roast, the meat is properly cooked when the temperature reaches that all-important 135 degrees.
For chefs who demand a time-based approach, popular recipes are a good resource. The renowned food blogger behind Smitten Kitchen suggests that cuts of beef such as roast beef be placed in a slow cooker on a low temperature setting for 8 to 10 hours, taking the guesswork out of internal temperature while also ensuring good taste.