I’ve recently had to manually cancel a build
It would make sense to add a build timeout (like for example one hour) that could be overridden through the snapcraft.yaml file
Please file a bug against Launchpad itself for this instead (it’s not something that build.snapcraft.io would handle directly).
Done: https://bugs.launchpad.net/launchpad/+bug/1844402
For anyone reading this: Please visit the bug and select that it effects you, if you also need this feature added