How long to warm taco bar dishes in a roaster
Well, if anybody gives you a solid figure on how long it's going to take, they almost certainly don't know what they're talking about. Even if they did know how hot your appliance gets and how quickly; the quantities of your ingredients, the initial temperature of your ingredients; the water content, fat content, and density are of your ingredients; how big the pieces of beef are, etc, providing anything more than a ballpark figure would be overly optimistic. In almost every circumstance, time is the least precise measurement to measure how cooked something is. 

A good temperature to shoot for with any buffet items is just above 140 degrees without going too far over. If your beef is rare or medium rare, be sure to heat it immediately before serving so it's not sitting around hot– it will become tough, dry, and overcooked quickly. Stirring makes a huge difference in both quality and reheating time, so be sure to consistently stir it up.