I use 30-6-20(after 4 cycles) pattern. You have to find yours through trial and error.
The goal is to not switch context. Social media is awfully bad in this.
If in work, I reply to Slack messages or message from family/friend- if any. They know to call in an emergency and not expect immediate reply to messages.
I have found drinking water and taking a quick stroll being the most effective pomodoro gap actvity. Although it is not always possible.