What's the service life of AK-47 in full-auto before it start to keyholing?
I only shoot AK-47 in full-auto for obvious fun reason... occasionally full mag & drum dump but mostly burst fire.
Usually I let it cool before it gets too hot.
Also, how much does it cost to rebarrel the gun?
Thanks in advance.

I would imagine it depends on which type of AK you have vs. how easy or hard it is to re-barrel.

Is it a milled or stamped receiver?

In general AK 7.62 barrels seem to last longer than M4 5.56 barrels as the pressure is a lot less 55K vs. 45K psi.

The best thread I am aware on the topic of long term AK performance is from Ron @ Battlefield Vegas.

https://www.ar15.com/mobile/topic.html?b=4&f=64&t=159106&page=1

In general it appears that eithers the trunnions and/or receivers crack or the gun goes out of headspace  prior to barrel keyholing, but you can get upwards of 100K rounds through an AK (depending upon manufacture origin) before something serious goes wrong with the receiver.

The most important aspect of how long your AK lasts seem to be more country or manufacture of origin that round count.