The weight capacity of a fake wall fireplace depends on the material it's made from and the design. The maximum load you can place on a 60" mantel is about 150 lbs. For a 72" mantel, 180 lbs. is roughly the most weight the fireplace can hold.
The material matters as well. A metal with textured stone faux fireplace could hold decorative items up to 85 lbs. An oak mantel with 3" thickness and a 60" length could comfortably hold up to 150 lbs.
The weight capacity of a fake fireplace wall depends on the installation too. For your mantel to hold as much weight as possible, reinforce with lag screws or install the mantel onto a backboard.