I really hate to break it to you but there is no such thing as a retired evil pc. if they are retired they are dead.
As to how soon it really depends on how creative the player can be. Even from an early level a Crusader could win through attrition vs this kind of army, the trick is to limit the number of attacks they can take per round and get DR against their attacks.
Quick and dirty no matter the class is some way to become immune to their primary attack forms, either through DR so high they can't bypass it or immunity to particular attack types. Some way of making them not have a chance at hitting you, ie invisibility and fly. A way of recovering your resources, mostly spells but possible for other things, rope trick or healing powers from ToB or other forms of permanent or infinite restoration.
Like was already stated. Kingdoms don't keep standing armies anymore it is just cheaper to hire a few mid level adventurers for a few thousand gold each than to equip an army for 10s of thousands of gold it would cost to equip them.