Some devs interrupt what they are doing when they see a PR in a Slack notification, most don't.
Most devs set aside some time at most twice a day for PRs. That's 5 hours at least.
Some PRs come in at the end of the day and will only get looked at the next day. That's more than 5 hours.
IME it's rare to see a PR get reviewed in under 5 hours.