Our full acceptance test run in 3 hrs (on build servers with 64 cores and 3xx GB memory) and we have PRs coming up every hour or so. We run the acceptance test

Our full acceptance test run in 3 hrs (on build servers with 64 cores and 3xx GB memory) and we have PRs coming up every hour or so. We run the acceptance tests for all PRs and also the main branch after each change. So a given PR needs to be 'green', and if we merge the PR then we run the tests on the integration branch too. If the integration branchs gets broken somehow, then all merges stop -- we don't merge any further PRs into a 'broken' branch. We have tried the option of always rebasing PRs on the most recent integration branch and re-running the tests, but that results in exponential number of builds. Of course you need a meaningful build farm -- we have around 1000 CPU cores for ~30 developers that are fully utilised during work hours.