Up to how fast can elk antlers grow?

Up to 1 inch (2.5 cm) per day! WOW Elk shed their antlers every late fall or winter and begin growing new antlers in late March or early April. For the first four months, the antlers are covered in nutrient-rich skin called velvet. The velvet is what allows the antlers to grow so fast. A mature bull rack can weigh over 500 pounds (225 kg)! At the beginning of August, the velvet begins to fall off and the fully grown antlers are ready for display...and combat. The antlers are true bone, not horns which are made from keratin.

https://www.elk101.com/2011/04/the-miracle-of-antler-growth/