There's no hard rule. The committers are expected to behave like a civilised human beings, ie talk to each other to reach consensus.

Let's say you have a fix for some driver used on ARM. You'll submit it via Phabricator to get ARM maintainers to review it. If they don't, because eg they don't have time, then you'll probably commit it anyway, but if someone points out you did something wrong, you'll be expected to fix it.