Several several years ago with our fall calving herd I creeped the calves with a free choice high quality corn gluten based feed. I put out the creep when they were about 1 month old. When they were around 6 mos old, I left them on the free choice feed after I weaned. I sold them around 45 days post weaning.
That was a VERY expensive lesson. My steers averaged 800+lbs, but the buyers sat on their hands and they got hammered for being too fleshy. In truth they were some butter balls.. looked like a bunch of finished hogs.
Now I hold off on creep until 30 days pre weaning, and they average 575-650.. much much more profitable (price-input+buyer's preference=profit)