The actual replenishment rates are determined in the EDB by unit name.
Here is an example of Gondor Infantry:
Code:
recruit_pool "Gondor Infantry" 1 0.08 2 0 requires factions { sicily, } and region_religion numenorian 66 and hidden_resource grasland and not hidden_resource amroth and not hidden_resource pelargir
recruit_pool "Gondor Infantry" 1 0.04 1 0 requires factions { sicily, } and region_religion numenorian 66 and hidden_resource forest and not hidden_resource amroth and not hidden_resource pelargir
recruit_pool "Gondor Infantry" 1 0.02 1 0 requires factions { sicily, } and region_religion numenorian 66 and hidden_resource desert or hidden_resource mountains
As can be seen they have the highest replenishment rate in grasland, followed by forest with half the rate and finally desert or mountains with one quarter of the grasland rate.
This can vary from unit to unit and also by facton type. The only way to be sure is to find the unit in question within the building that is producing it and obtaining the replenishment rate.