At first glance it really looks like this is a hard-coded value. There is a "MORDOR_LUMBERMILL_MONEY_TIME" macro in gamedata.ini, but it doesn't seem like it's used. It seems connected to the SpecialAbilityHarvest special power. Maybe someone else managed to change it somehow, but I don't see an obvious way to modify that value.
Also, for coding-related issues we don't need to know your system information, it's not relevant
I figured more info was better than less. I'll look into the SpecialAbilityHarvest power. Thanks