Factors I think come into play:
1) How accurate was it to start with?
A 0.25moa rifle might show visible losses after 40 rounds, but you might not notice any degradation in a 1moa rifle for 300.
2) The chambering and type of barrel have an impact. A rimfire with a lapped barrel will go hundreds of rounds with no change while a rough button rifled .300wm might foul up in 40 rounds.
Personal experience:
My 1moa factory-barreled “normal” rifles start to get loose around 120 rounds. At least that’s where I begin to notice.
My precision rimfire will go at least 300 rounds before it starts to open up and I need to scrub just a few inches past the chamber.
My single-point cut rifled, lapped 5.56 SR that shoots 1/2moa starts to build excessive pressure from carbon ring before I notice accuracy falling off, and that happens around 130 rounds.
So my policy is 120-140 rounds as a general rule.