The time required to train one soldier depends on Garrison level, Alliance level and the Governor.
Archers, due to their long ranged weapons, receive a 'first attack round' when fighting other troops. In this round the archers deal around half their total damage to the enemy without receiving any damage in return. Many strategies for low level wildlands involve killing all opposing troops with this attack before they have the chance to return fire.
Using a legend with low courage, it takes roughly 2x the archers to One-Turn Kill (OTK) halberdiers, 4x the archers for swordsmen, and 14x for charioteers. If your legend's courage is its main stat, fewer archers are needed. For a closer approximation of needed numbers of archers, see: Archers vs Chariots in Wildlands