after X number of rounds what parts should be replaced?
20k or 30k do you replace anything in particular??? Any rules of yours that you apply to all rifles?
On an AK? Either it goes bang and sends a bullet in the general vicinity of your target, or it does not. There's not much in-between on an AK.
The springs are made from several strands of wound wire which are so superior to single-wire springs that they should last nearly indefinitely. The other parts are made of steel or wood and should last a good long time too. I'd just grease the rails and the bolt lugs occasionally and keep clanging away until it ceases to fire.
Replace parts if they're worn or broken.
Lubricate the rifle properly and worn parts aren't likely.
Any part can break, but the rifle didn't earn it's reputation for reliability by breaking parts frequently.
I would absolutely replace the recoil spring before that. It may still work but you are beting the rifle up more with a weak spring.