That's in the EDB. TATW's is particularly complicated to allow for the different faction's productivity levels based on the regional terrain. So it will take editing multiple entries for each level of building. First I would check in your game that this isn't what is causing your grief as it is intentional. Orcs have 100% replenishment in mountains, 75% in grassland, and 50% in desert or forest; better prospects than the good factions.
But if you still want to change it, go to the data folder he mentioned and find export_descr_building. Scroll down until you find the building which recruits the unit you want to change. Snagas should be in the barracks section.
Code:
town_watch city requires factions { sicily, byzantium, milan, scotland, turks, spain, venice, england, france, hre, portugal, egypt, mongols, moors, }
{
convert_to 1
capability
{
recruit_pool "High Sword" 1 0.12 2 0 requires factions { egypt, } and region_religion orthodox 33 and hidden_resource forest
This is an example straight from the file. Find the unit you want. You see the name* there in quotes. There's a bunch of other stuff but all you need is the second number there in red. It shows what portion of a unit replenishes every turn. A value of 1 will have a new unit available very turn, .5 every two turns, the current value of .12 would require ~9 turns.
*The name given here is required to match the name given in the export_descr_unit file(EDU) and not the one given in game, so be warned, they occasionally vary.
EDIT: As a general rule, whenever you edit anything you should back up the file you plan to alter, just in case. Hope this helps.