Hi,
Standard is approx 3m from the ground up but all APs have antenna power ratings that can be increased to give signal boost as per requirement. Moreover, I always recommend to use Huawei WLAN Planner tool as there are numorous other factors that play a role in this; like AP height is around 6m but there are no interference so signals will reach easily to the clinet. However, there are cases where ceiling height is low but there are interference like walls, partitions etc are there so it decrease the overall signal quality.
So, when you use WLAN Planner, you can set the AP height, all the interference, wall types, inter-floor distance etc so you get a concrete and well-planned blanket coverage with best AP locations.
I hope it helps and do let me know in case you need to discuss this in detail.