It depends on the cut of beef and how much fat content there is. Leaner cuts that have little fat or are grass fed beef cuts that are naturally leaner will cook much faster than say a rib-eye that has more fat. It also depend on how you like your beef: well done, medium, or rare or something in between. For beef with a moderate amount of fat the rule of thumb I use is 3 min per side for rare; 4 min per side for medium; 8 min per side for well-done.