If your barrel doesn't touch nothing with it gets hot AND you face the receiver to make it square with the receiver threads, it shouldn't walk when it gets hot.
You can easily get through a 80-100 shot match without cleaning.
Don't clean if you don't plan on practicing before a match.
How long before accuracy of a barrel before it goes south depends on too many things, each gun is different.
Each barrel is different. On my Super Match M1A I used Barnett barrels, one barrel I only got about 4000 rounds, another 5000 plus. They were the same barrels and loads.
That's why its critical to keep good Data or Score books so you can keep track.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071